How Much Force is Too Much? Understanding Proportionality in Japanese Self-Defense Law

Japanese Attorney at Law - Bengoshi L.L.

02 Jun 2025 • 10 min read

The right to self-defense (正当防衛 - seitō bōei) is a well-established justification in Japanese criminal law, permitting individuals to use force to protect themselves or others from harm. However, this right is not absolute. Beyond the requirements that an attack be "imminent and unjust" and that the defender act with an "intent to defend," Japanese law imposes a crucial third condition: the defensive act must have been "unavoidably performed" (やむを得ずにした行為 - yamu o ezu ni shita kōi). This phrase encapsulates the principle of proportionality or reasonableness, essentially asking whether the amount and type of force used by the defender was appropriate in light of the threat faced.

Navigating this standard can be complex. How much force is considered too much? When does a defensive act cross the line from justified protection to unlawful excess? This article delves into the concept of proportionality in Japanese self-defense law, examining the factors courts consider and analyzing a key Supreme Court of Japan decision that provides significant guidance on this often-litigated issue.

The "Unavoidably Performed Act" and the Principle of Proportionality (相当性 - Sōtōsei)

Article 36, paragraph 1 of the Japanese Penal Code exempts from punishment "an act unavoidably performed to protect the rights of oneself or any other person against an imminent and unjust infringement." The phrase "unavoidably performed" is the statutory anchor for the principle of proportionality, often referred to in Japanese legal discourse as 防衛行為の相当性 (bōei kōi no sōtōsei – the reasonableness or adequacy of the defensive act).

Core Meaning and Judicial Interpretation

This requirement means that the defensive measures taken must be both necessary to repel the unlawful attack and must not substantially exceed what is reasonably required by the specific circumstances. It is a balancing act, assessed objectively, considering the nature of the threat and the means of defense.

It's important to note that "unavoidably performed" does not rigidly mean that the defender must have used the absolute least amount of force conceivable. The law recognizes that individuals acting under the duress of an attack cannot be expected to make perfectly calibrated, split-second judgments about the precise quantum of force needed. The focus is on whether the defensive response was, broadly speaking, a reasonable and necessary reaction to the danger presented.

The Supreme Court of Japan has provided key interpretations of this standard over the years:

In a decision on August 18, 1949 (Saikō Saibansho Hanketsu, Shōwa 24-nen 8-gatsu 18-nichi), the Court defined an "unavoidably performed act" as one that, under the specific circumstances of the case, can be recognized as natural and appropriate as a defensive act according to prevailing social norms (当時の社会通念が防衛行為として当然性、妥当性を認め得るもの - tōji no shakai tsūnen ga bōei kōi toshite tōzensei, datōsei o mitome uru mono).
Later, in a decision on December 4, 1969 (Saikō Saibansho Hanketsu, Shōwa 44-nen 12-gatsu 4-nichi, Keishū 23-kan 12-gō 1573-ページ), the Supreme Court elaborated that an "unavoidably performed act" signifies that the counterattack was within the "minimum necessary limits" (必要最小限度 - hitsuyō saishō gendo) as a means of defense. In other words, the defensive act must possess "proportionality" (相当性 - sōtōsei). The Court further clarified that as long as the defensive act does not exceed this limit of proportionality, it remains justified self-defense even if the harm inflicted on the aggressor happens to be greater than the harm initially threatened to the defender's interest.

These rulings establish that proportionality is a central component of the "unavoidably performed" requirement, judged by an objective standard based on social norms and the necessity of the defensive response in the face of the specific threat.

Factors Considered in Assessing Proportionality

Determining whether a defensive act was proportionate is a highly fact-specific inquiry. Japanese courts consider a range of factors, including:

Nature and Severity of the Attack: The perceived dangerousness of the aggressor's actions is paramount. Was it an unarmed physical assault, or did it involve a weapon? Were the threats verbal or accompanied by overt physical actions?
Means Used by the Attacker and the Defender: This involves a "balance of arms" consideration. Using a deadly weapon against an entirely unarmed attacker, or employing force far exceeding that of the aggressor, will be heavily scrutinized and is likely to be deemed disproportionate unless other compelling circumstances exist.
Relative Physical Characteristics of the Parties: Significant disparities in age, gender, physical build, strength, or specialized skills (e.g., martial arts training) between the attacker and the defender are crucial. A physically weaker person may be justified in using more force or even a weapon against a stronger, unarmed assailant. The number of attackers versus defenders is also a critical factor.
Location and Circumstances of the Confrontation: The environment in which the defense occurs plays a role. Was it a confined space with no escape routes, or an open area? Did it occur in public or private? What time of day was it? These factors can influence the defender's perception of danger and the reasonable options available.
The "Necessity" of the Defensive Act: While Japanese law does not impose a strict, universal "duty to retreat" before using defensive force, courts do consider whether less harmful means of averting the threat were reasonably available and apparent to the defender. If a minor threat could have been easily neutralized or avoided without resorting to significant force, the use of such force might be deemed to exceed the bounds of necessity and proportionality.
The Interest Protected Versus the Harm Inflicted: As the 1969 Supreme Court ruling indicated, a strict balancing of the value of the interest protected against the harm inflicted on the aggressor is not required for justified self-defense. One may be justified in using significant force to protect their life or bodily integrity even if it results in severe injury or death to the aggressor. However, a gross disproportion (e.g., using deadly force to prevent a very minor theft of low-value property or a trivial physical touch) would likely lead to a finding that the defensive act was not "unavoidably performed" and thus exceeded the bounds of proportionality.

The Parking Lot Dispute: Supreme Court Decision, November 13, 1989

A leading Supreme Court case that meticulously examined the proportionality of defensive actions is the decision of November 13, 1989 (Saikō Saibansho Hanketsu, Heisei Gan-nen 11-gatsu 13-nichi, Keishū 43-kan 10-gō 823-ページ).

Factual Background

The defendant had parked his light cargo vehicle near an empty lot while attending to business at a nearby pharmacy. Another driver, K, operating a dump truck, found the defendant's vehicle obstructing his attempt to enter the lot. After an exchange where K yelled at the defendant, and the defendant moved his vehicle, the defendant, annoyed by K's aggressive and rude demeanor, told K to be more careful with his language.

This incensed K. After parking his dump truck, K aggressively approached the defendant, shouting, "You want to get hit?" while making punching and kicking gestures. The defendant, perceiving K to be younger and physically superior, became frightened and began backing away towards his own vehicle. K continued to pursue him closely.

As he retreated, the defendant remembered he had a vegetable-peeling knife (nakiri bōchō with a blade length of approximately 17.7 cm), which he used for tasks like peeling fruit, in his vehicle's console box. In a split-second decision to deter K's imminent assault and protect himself, the defendant circled his vehicle, reached in through the open window, and retrieved the knife. He held the knife by his hip.

Facing K, who was then about three meters away, the defendant said, "If you can hit me, try it!" K, apparently undeterred, advanced a few more steps, retorting, "If you're going to stab me, try it!" The defendant then escalated his warning, saying, "You want to get cut?"

For these actions—displaying the knife and uttering these threats—the defendant was charged with intimidation with a dangerous weapon (under the Act on Punishment of Violent Acts, etc.) and unlawful carrying of a bladed tool (under the Swords and Firearms Control Act). His defense was that his actions constituted legitimate self-defense.

Lower Court Rulings

The Summary Court (first instance) rejected the self-defense claim. It found that there was no imminent threat before the defendant retrieved the knife. Instead, it concluded that the defendant had anticipated a physical altercation and armed himself to gain an unfair advantage, essentially viewing his act of retrieving the knife as a preemptive aggressive move.
The High Court took a different view. It acknowledged that K's aggressive advance did constitute an "imminent and unjust infringement" and that the defendant acted with the "intent to defend." However, the High Court found the defendant's actions to be disproportionate. It reasoned that confronting K—who, at that point, was unarmed and only making threatening gestures—with a 17.7 cm kitchen knife exceeded the bounds of reasonable self-defense. Therefore, the High Court found the defendant guilty of excessive self-defense (過剰防衛 - kajō bōei), a determination that mitigates punishment but does not fully excuse the conduct.

The Supreme Court's Reasoning on Proportionality

The Supreme Court reversed the High Court's decision. It concluded that the defendant's actions did not exceed the bounds of proportionate self-defense and, therefore, constituted fully justified self-defense, leading to an acquittal on the intimidation charge (the illegal carrying charge was also dismissed as the carrying was justified by the self-defense situation). The Supreme Court's reasoning focused on the following points:

Defendant's Consistently Defensive Posture: The Court emphasized that the defendant's actions throughout the encounter were fundamentally defensive. He was retreating from K's aggression and was being actively pursued. He only retrieved the knife when K was closing in on him, creating a situation of immediate peril.
The Nature of K's Threat: K was clearly the aggressor. He was younger, physically superior to the defendant (a factor noted by the Supreme Court), and was advancing with explicit verbal threats ("You want to get hit?") and menacing physical gestures (punching and kicking motions). This constituted a credible threat of imminent physical assault.
Defendant's Use of the Knife as Deterrence: Crucially, the Supreme Court noted how the defendant used the knife. He did not immediately attack K upon retrieving it. Instead, he held it by his hip—a defensive or cautionary posture—and issued verbal warnings ("If you can hit me, try it!", "You want to get cut?"). His actions were aimed at deterring K's further advance and preventing the threatened assault, rather than launching an offensive attack himself.
No Obligation to Absorb an Assault: The Court implicitly recognized that the defendant was not legally obligated to wait until he was actually struck by the physically superior K before taking measures to protect himself. In the face of K's relentless and aggressive advance, the display of a weapon to deter an imminent assault was seen as a legitimate defensive tactic.
Holistic Assessment of Proportionality: Considering all these factors—K's aggression and physical superiority, the defendant's retreat and defensive posture, and his use of the knife primarily as a deterrent accompanied by warnings—the Supreme Court concluded that the defendant's actions were a proportionate response to the imminent threat he faced. The act of retrieving and displaying the knife was deemed "unavoidably performed" to protect himself from K's impending assault.

Significance of the 1989 Supreme Court Case

This 1989 decision is highly significant for several reasons:

Clarification on Weapon Use Against Unarmed Aggressors: It establishes that displaying a weapon, even one with lethal potential like a kitchen knife, can be a proportionate defensive measure against an unarmed but physically superior and aggressively advancing attacker, provided the weapon is used primarily for deterrence and not in an immediately offensive or excessive manner.
Importance of Defender's Overall Conduct: The ruling underscores the importance of the defender's entire course of conduct in the proportionality assessment. The defendant's initial retreat and subsequent verbal warnings while holding the knife defensively were key factors.
"Balance of Arms" is Not Merely Mechanical: It refines the "balance of arms" consideration, showing that a purely mechanical comparison of whether both parties were armed is insufficient. The physical capabilities, age, and aggressive demeanor of the parties are also critical elements in assessing the overall balance of threat and necessary defense.
Defensive Display vs. Offensive Use: The case implicitly distinguishes between the defensive display of a weapon to ward off an attack and the offensive use of that weapon to inflict harm. The former is more likely to be deemed proportionate in circumstances like those faced by the defendant.

Excessive Self-Defense (過剰防衛 - Kajō Bōei)

When a defensive act, though prompted by an imminent and unjust infringement and performed with defensive intent, goes beyond what is reasonably proportionate to the threat, it constitutes "excessive self-defense" (Article 36, paragraph 2 of the Penal Code).

Legal Effect: Unlike justified self-defense which results in non-punishment, excessive self-defense does not fully excuse the act. However, the law mandates that the punishment for the underlying crime (e.g., assault, injury, homicide) may be reduced or, in certain circumstances, even remitted entirely. If the excess was "due to fear, surprise, excitement, or confusion" (恐怖、驚動、興奮又は狼狽に因りて - kyōfu, kyōdō, kōfun matawa rōbai ni yorite), the punishment must be remitted (i.e., no punishment is imposed), effectively leading to an acquittal despite the excess.
Drawing the Line: The distinction between justified self-defense and excessive self-defense can be very fine and is intensely fact-dependent. The 1989 Supreme Court case was ultimately found to be justified, whereas the High Court in the same case had deemed it excessive, illustrating the potential for differing judicial assessments.

The "Weapons Parity" Consideration in Practice

While not a formal, codified legal rule, Japanese courts often implicitly or explicitly consider the principle of "weapons parity" (武器対等の原則 - buki taitō no gensoku) when evaluating proportionality. Generally, using a deadly weapon against an entirely unarmed attacker raises a strong inference of disproportionality.

However, the 1989 Supreme Court case demonstrates that this is not an absolute bar to justified self-defense. If other circumstances—such as a significant disparity in physical strength favoring the unarmed attacker, the attacker's extreme aggression, and the defender's use of the weapon primarily for deterrence rather than immediate offensive assault—are present, the defensive use of a weapon can still be deemed proportionate. Legal commentaries on the 1989 case often highlight how academic reviews of earlier case law (such as analyses by Professors Ōkoshi Yoshihisa and Kōjō Toshimaro mentioned in some case discussions) had already noted trends where:

Proportionality was generally found in unarmed vs. unarmed, or weapon vs. similarly dangerous weapon scenarios.
An unarmed defense against a weaponized attack was typically considered proportionate.
Weaponized defense against an unarmed attack was the classic scenario for finding excessiveness (disproportionality).
However, even in the last scenario, exceptions were recognized if no other means of defense were realistically available to the defender, especially if outmatched.

The 1989 Supreme Court decision can be seen as a significant judicial affirmation and refinement of this latter exception, emphasizing that a holistic view of the entire confrontation, including the physical dynamics between the parties and the defender's overall defensive intent and conduct, is necessary. It moved beyond a simple mechanical comparison of whether weapons were present on both sides.

Conclusion

Determining whether the force used in self-defense was "too much" under Japanese law is a nuanced and highly contextual exercise. The principle of proportionality, embedded in the "unavoidably performed act" requirement of Article 36(1) of the Penal Code, demands a careful balancing of various factors. There is no rigid formula; courts meticulously weigh the nature of the threat, the means of defense employed, the relative capabilities of the parties, and the overall circumstances of the encounter.

The Supreme Court's 1989 decision in the parking lot dispute case is a crucial guidepost. It illustrates that the display and threatened use of a potentially lethal weapon against an unarmed but physically superior and aggressive assailant can fall within the bounds of proportionate and justified self-defense, particularly when the defender is clearly acting to deter an imminent assault and not as the primary aggressor. This ruling underscores that Japanese law, while seeking to prevent excessive violence, recognizes the right of individuals to take necessary and reasonable measures to protect themselves from immediate harm, adapting the assessment of proportionality to the stark realities of the specific confrontation.