Proving Negligence (Kashitsu) in Japanese Tort Law: What is the Standard of Care, Especially in Medical Malpractice?

In Japan, as in most legal systems, establishing negligence (過失 - kashitsu) is a cornerstone for holding a party liable in tort for unintentional harm caused to another (governed generally by Article 709 of the Civil Code). Whether in the context of a traffic accident, a defective product, or alleged professional malpractice, the question of whether the defendant acted negligently is often the central point of contention. For businesses and professionals, understanding how Japanese law defines and assesses negligence, particularly the applicable standard of care, is crucial for risk management and dispute resolution.

This article explores the concept of negligence in Japanese tort law, focusing on its objective nature, the key elements of foreseeability and the duty to avoid harm, and how the standard of care is determined, with a special look at the complexities involved in medical malpractice cases.

1. The Modern Understanding of Negligence (Kashitsu) in Japan

While older legal theories sometimes viewed negligence subjectively (as a personal lack of due attention), the prevailing modern understanding in Japanese law, supported by long-standing case law such as the Great Court of Cassation judgment of December 22, 1916 (the "Osaka Alkali" case), is an objective one. Negligence is generally defined as the breach of an objectively determined duty of care, where such breach leads to foreseeable harm.

This objective approach typically involves two key elements:

1. Foreseeability of Harm (結果予見可能性 - Kekka Yoken Kanōsei):
   The first question is whether a reasonably prudent person, in the same position and circumstances as the defendant, could have foreseen that their act or omission might result in the harmful outcome that occurred. If the harm was entirely unforeseeable, there is generally no basis for finding negligence.
2. Breach of the Duty to Avoid Harm (結果回避義務違反 - Kekka Kaihi Gimu Ihan):
   If the harm was foreseeable, the next question is whether the defendant failed to take reasonable measures that a prudent person would have taken to avoid that harm. This "duty to avoid harm" is not a duty to prevent all possible harm, but rather a duty to exercise a certain level of care.

The benchmark for assessing both foreseeability and the adequacy of avoidance measures is the "reasonably prudent person" (or, in professional contexts, the "reasonably prudent professional"). This is an abstract, objective standard.

2. The Standard of Care – General Principles

The specific standard of care required in any given situation is not fixed but is determined objectively based on the particular circumstances. Factors that courts consider when evaluating whether the duty of care was breached include:

• The severity of the potential harm.
• The probability of the harm occurring.
• The cost, burden, and feasibility of taking preventive measures. (While not explicitly adopting the "Hand Formula" as a rigid mathematical test, Japanese courts often engage in a similar balancing of these considerations).
• The social utility of the defendant's conduct.

Furthermore, statutes, regulations, and established industry customs or best practices can play a significant role in defining the expected standard of care. Compliance with such standards may be evidence of due care, while non-compliance can be strong evidence of negligence, though neither is necessarily conclusive on its own.

3. Standard of Care in Professional Liability – The Case of Medical Malpractice

Medical malpractice litigation is a prominent area where the standard of care is intensely scrutinized. The general principles of negligence apply, but they are adapted to the specific context of medical practice.

• The "Medical Standard" (医療水準 - Iryō Suijun):
  The primary benchmark for assessing a physician's conduct is the "medical standard practiced in clinical medicine at the time of the diagnosis and treatment" (診療当時のいわゆる臨床医学の実践における医療水準 - shinryō tōji no iwayuru rinshō igaku no jissen ni okeru iryō suijun). This was established by the Supreme Court judgment of March 30, 1982 (Hanrei Jihō No. 1039, p. 66).
• Not a Single National Standard: Importantly, this medical standard is not a uniform, nationwide standard applicable to all medical institutions equally. The Supreme Court judgment of June 9, 1995 (Minshū Vol. 49, No. 6, p. 1499) clarified that the expected level of care can vary depending on factors such as:
  • The nature, function, and resources of the medical institution (e.g., a cutting-edge university hospital with specialized departments and advanced technology is generally expected to meet a higher standard in complex cases than a small, local clinic with limited resources).
  • The medical environment of the particular region.
• Doctor's Discretion in Treatment Choice:
  If multiple recognized and medically sound treatment options exist that are consistent with the applicable medical standard, a physician generally has the discretion to choose among them in consultation with the patient. The choice of one accepted method over another is not, in itself, negligent, provided the chosen treatment is then performed competently and with due care.
• Individual Skill vs. Objective Standard:
  The Supreme Court judgment of January 23, 1996 (Minshū Vol. 50, No. 1, p. 1) established that a physician's personal, exceptional skills do not elevate the legally required standard of care beyond what is objectively expected for their type of institution and the circumstances. Conversely, a physician's lack of skill or experience is generally not an excuse if they fail to meet the objective medical standard applicable to the situation. Patients are entitled to expect a certain baseline level of competence. Similarly, a patient's specific wishes or expectations do not, in themselves, alter the objective medical standard against which the physician's conduct is judged.

4. Specific Issues in Proving Medical Negligence

Proving negligence in medical malpractice cases can be particularly challenging due to the complexity of medical science and the information asymmetry between medical professionals and patients.

• Duty to Explain (Informed Consent - 説明義務, Setsumei Gimu):
  While the duty to explain risks, benefits, and alternatives to a patient (leading to informed consent) is often considered a distinct duty, a breach of which can itself be a tort, it is also closely related to negligent treatment. Failure to properly inform a patient, especially if it leads to the patient choosing a course of action they would have otherwise avoided and suffering harm as a result, can be a component of overall negligent medical care.
• Causation: Even if negligence (a breach of the medical standard) is established, the plaintiff must still prove a causal link between that negligence and the adverse outcome. This is often a major hurdle in medical cases.
• Role of Medical Records and Expert Testimony: Medical records are critical pieces of evidence. Due to the specialized nature of medical issues, expert testimony from other medical professionals is almost always indispensable in evaluating the appropriateness of the defendant physician's conduct and the cause of the patient's injury.

5. "Presumption" or Strong Inference of Negligence in Certain Contexts

While the ultimate burden of proving negligence generally rests with the plaintiff, Japanese courts, particularly in medical and pharmaceutical contexts, may draw strong inferences of negligence from certain facts, effectively shifting the practical burden of explanation to the defendant.

• Failure to Adhere to Drug Package Inserts (添付文書 - Tenpu Bunsho):
  A significant line of case law, including the Supreme Court judgment of January 23, 1996 (Minshū Vol. 50, No. 1, p. 1), holds that if a physician deviates from the instructions, warnings, or contraindications contained in a pharmaceutical product's official package insert (tenpu bunsho) without a specific, medically justifiable reason for doing so, their negligence is often strongly inferred or "presumed."
  • Rationale: Package inserts are considered a primary and authoritative source of information provided by drug manufacturers to physicians regarding the safe and effective use of medications. They reflect a distillation of clinical trial data and expert knowledge.
  • This "presumption" is not a formal legal presumption that shifts the ultimate burden of proof for a principal fact (as negligence itself is a legal evaluation of facts). Rather, it's a judicial practice where non-adherence to such critical guidance creates a strong prima facie case of negligence. The physician then bears a heavy burden to demonstrate that their deviation was reasonable and medically appropriate under the specific circumstances.
• Academic Literature vs. Package Inserts:
  If warnings or critical information about a drug or procedure are present only in academic literature (especially if not yet widely disseminated or incorporated into official guidelines or package inserts), failure to follow such information is less likely to trigger the same strong inference of negligence. The authoritativeness, general acceptance within the medical community, and accessibility of the information in the academic source would be key factors. A physician is generally expected to keep up with significant developments in their field, but not necessarily every single piece of emerging research that hasn't yet been validated or adopted into standard practice or official warnings.

The example provided in the PDF materials (item 148) concerning a pharmaceutical company's liability for vaccine side effects touches on similar themes: if known risks are comparable to existing alternatives and fall within an accepted range, negligence might be hard to establish. However, if there was foreseeable potential for more severe or unique side effects (perhaps indicated in specialized journals or early overseas reports), the duty of care might include further investigation or warnings, and failure to do so could be negligent. The level of diligence required would be assessed against the standards of a reasonable pharmaceutical manufacturer.

6. Proving Negligence in Business and Other Contexts

While medical malpractice provides a detailed illustration, the objective standard of care based on foreseeability and the duty to avoid harm applies broadly across all types of tort claims requiring proof of negligence in Japan.

• Businesses are expected to exercise reasonable care in all aspects of their operations to prevent foreseeable harm to customers, employees, and the general public. This includes areas like product safety design and manufacturing, premises liability (ensuring a safe environment for visitors), and managing environmental impacts.
• Compliance with relevant laws, regulations, and established industry best practices is often strong evidence of having met the standard of care. Conversely, violating such standards can be compelling evidence of negligence. However, compliance is not always a conclusive shield, nor is non-compliance automatically negligence; the overall reasonableness of the conduct in the specific circumstances remains the ultimate test.

Conclusion: A Contextual and Objective Standard

Proving negligence (kashitsu) in Japanese tort law is a fact-intensive process that centers on demonstrating a breach of an objective duty of care. This duty is defined by what a reasonably prudent person (or professional) would have foreseen and done in similar circumstances to avoid causing harm. While general principles of foreseeability and the duty to avoid harm are consistently applied, the specific standard of care is highly contextual and tailored to the situation at hand. In specialized fields like medicine, established "medical standards," professional guidelines, and adherence to critical safety information (such as drug package inserts) play a particularly significant role in determining whether conduct was negligent. For businesses and individuals alike, a proactive approach to identifying foreseeable risks and implementing reasonable preventive measures is key to meeting the expected standard of care and mitigating potential tort liability in Japan.