Navigating Patent Infringement in Japan: What Are the Requirements for the Doctrine of Equivalents?

In the realm of patent enforcement, the precise wording of patent claims defines the boundaries of a patentee's exclusive rights. However, strict adherence to literal claim language can sometimes allow infringers to make minor, insubstantial changes to a patented invention and thereby evade liability, undermining the patent system's objective of rewarding and protecting innovation. To address this, many jurisdictions, including Japan, have adopted the Doctrine of Equivalents. This doctrine allows a court to find infringement even when the accused product or process does not fall within the literal scope of the patent claims, provided certain conditions are met. Understanding the nuances of how this doctrine is applied in Japan is critical for businesses involved in patent litigation or seeking to enforce their patent rights in the Japanese market.

The Genesis and Rationale of the Doctrine of Equivalents in Japan

The fundamental principle for determining the technical scope of a patented invention in Japan is laid out in Article 70, Paragraph 1 of the Japanese Patent Act, which states that the scope "shall be determined based on the statements in the scope of claims attached to the application." While this establishes the primacy of the claim language, an overly rigid application could render patent protection ineffective against trivial modifications.

Recognizing this, the Supreme Court of Japan, in its landmark decision on February 24, 1998 (often referred to as the Ball Spline Bearing case, Heisei 6 (o) No. 1083), formally endorsed the Doctrine of Equivalents in Japan. The Court acknowledged that if claim interpretation were strictly limited to the literal text, it could allow third parties to easily circumvent patents through minor alterations, thereby unjustly diminishing the value of patent rights and potentially discouraging innovation. The doctrine, therefore, serves to ensure fair and effective protection for patentees against such circumvention.

The Five Requirements for Applying the Doctrine of Equivalents in Japan

The Ball Spline Bearing decision established a five-part test for the application of the Doctrine of Equivalents. An accused product or process that differs from the literal claim language may still be found to infringe if all five of the following requirements are satisfied:

1. The Differing Element is Not an Essential Part of the Patented Invention (非本質的部分 - hi-honshitsuteki bubun)

The first requirement is that the element in the accused product or process that differs from the claimed invention must not be an "essential part" of the patented invention. There has been some evolution in how this "essential part" is identified:

• Early Approaches (e.g., "Osaka Method"): Some earlier interpretations involved dissecting the claim into essential and non-essential elements, with equivalence only being considered for substitutions of non-essential parts. This approach, however, proved problematic as it could lead to overly rigid outcomes.
• Prevailing Approach ("Tokyo Method" / Technical Idea Approach): The dominant view, supported by most court decisions including more recent ones, focuses on whether the substitution in the accused product alters the core "technical idea" or inventive concept underlying the patented invention. If the accused product, despite the substitution, still operates based on the same fundamental technical principle as the patented invention to solve the same problem, the substituted part is considered non-essential. Conversely, if the substitution changes the core technical idea or working principle, it is deemed an essential part, and the doctrine of equivalents cannot apply. A key decision from the Intellectual Property High Court on March 25, 2016 (the Maxacalcitol case, Heisei 27 (ne) No. 10014) reinforced this "technical idea" approach, emphasizing the need to prevent free-riding on the disclosed inventive concept.

The "essential part" is generally identified by examining the technical idea disclosed in the patent specification, particularly the problem the invention solves and the means by which it solves it.

2. Substitutability: The Substituted Element Achieves the Same Object and Effect (置換可能性 - chikan kanōsei)

The second requirement is that replacing the differing element in the patented invention with the corresponding element in the accused product or process must still allow the invention to achieve its intended object and produce substantially the same effects. This focuses on the functional and resultant equivalence of the substituted element.

If the substitution leads to a failure to achieve the invention's purpose or results in different operational effects, this requirement is not met. This also implies that if the specification itself describes a solution that, in reality, cannot achieve the stated object or effect even with the claimed elements, the premise for finding equivalence might be undermined.

3. Ease of Substitution at the Time of Infringement (置換容易性 - chikan yōisei)

The third requirement is that the substitution must have been easily conceivable by a person of ordinary skill in the art (a "POSITA") at the time of the infringing act (not at the time of the patent application). This "ease of substitution" is assessed from the perspective of a POSITA at the moment the infringement occurred.

The Supreme Court in the Ball Spline Bearing case explicitly adopted the "time of infringement" as the benchmark for this assessment. The rationale is that the doctrine aims to prevent easy circumvention that would be apparent to a skilled person looking to copy the invention at the time they are doing so. If a substitution would not have been obvious to a POSITA at that time, applying the doctrine could unfairly surprise third parties.
The definition of "POSITA" in this context is also relevant. Some scholars suggest that for inventive step (non-obviousness) assessments, the POSITA is typically considered from a research and development perspective, whereas for enablement and potentially for the doctrine of equivalents, the POSITA might be viewed from a manufacturing or implementation perspective. This distinction, though not always explicitly made by courts, could influence the assessment of what is "easily" substitutable.

4. The Accused Product/Process Itself Must Be Patentable over Prior Art (仮想的クレームの新規性・進歩性 - kasōteki kurēmu no shinkisei/shinposei)

The fourth requirement, often referred to as the "hypothetical claim" test or "ensnarement" limitation, dictates that the accused product or process, if it had been claimed at the time the original patent application was filed, must itself have been patentable over the prior art. In other words, the doctrine of equivalents cannot extend protection to cover subject matter that would have been unpatentable (e.g., for lacking novelty or inventive step) had it been explicitly claimed by the patentee.

This ensures that the doctrine does not allow a patentee to effectively claim, through equivalence, what they could not have obtained directly from the patent office. For example, if an allegedly equivalent product was itself known or obvious from prior art existing at the patent's filing date, this fourth requirement would not be met. This has been applied not only in the context of novelty and inventive step but also concerning other patentability requirements, such as the enlarged scope of a prior application under Article 29-2 of the Patent Act (as seen in a 2010 bar examination question context).
The burden of proof for this requirement is a point of discussion. While some court precedents suggest the alleged infringer bears the burden of showing the hypothetical claim would be unpatentable, some academic views argue that once relevant prior art is raised, the patentee should bear the burden of proving patentability.

5. No Estoppel Based on Prosecution History (審査経過禁反言 - shinsa keika kinpangen)

The fifth and final requirement is that there must be no "special circumstances," such as the accused product or process being something that was intentionally excluded from the scope of claims during the patent prosecution process (e.g., through arguments or amendments made to overcome prior art rejections). This is the principle of prosecution history estoppel (also known as file wrapper estoppel).

If the patentee narrowed their claims or made assertions during prosecution that led the patent office to grant the patent, they cannot later recapture that surrendered subject matter through the doctrine of equivalents. The rationale is to prevent patentees from taking contradictory positions and to uphold the integrity of the examination process.

• Scope of Estoppel (Flexible Bar vs. Complete Bar): The prevailing view in Japan is the "flexible bar" approach, meaning estoppel applies only to amendments or arguments that were material to the patent's grant, rather than a "complete bar" that would estop any claim to equivalents for an amended element regardless of the reason for amendment. The focus is on whether the applicant's actions could have influenced the examiner's decision on patentability.
• Applicable Proceedings: Prosecution history estoppel can arise not only from the initial examination but also from statements or amendments made during subsequent proceedings, such as invalidity trials.

Specific Issues and Advanced Topics in Japanese Doctrine of Equivalents

Beyond the five core requirements, certain specific scenarios raise additional considerations:

1. Equivalency for Elements Existing at the Time of Filing

A question sometimes arises whether the doctrine of equivalents can apply to an element in the accused product that was known and available as an alternative at the time the patent application was filed, but which the patentee did not explicitly claim. Some argue that if the alternative was known, the patentee should have claimed it, and the doctrine should not rescue such an omission. However, the prevailing view in Japan, reflecting the difficulty of drafting perfectly comprehensive claims and the structural advantage of infringers who can design around literal language post-issuance, is that the mere existence of the alternative at the time of filing does not automatically preclude a finding of equivalence. The focus remains on the five requirements.

2. Inconsistency Between Specification and Claims (The "Dedication Rule" Analogue)

A more specific version of the above issue arises when an alternative element is disclosed in the specification but not included in the claims. This is somewhat analogous to the "dedication rule" in U.S. patent law. If the patentee describes an alternative but deliberately claims more narrowly, there's a stronger argument that the unclaimed alternative was intentionally excluded or dedicated to the public.
While not a rigid rule, if the public, upon reading the specification and the narrower claims, would reasonably believe that the disclosed but unclaimed elements were not intended to be covered, a court might be less inclined to find equivalence for those elements, potentially viewing it as a form of conscious exclusion, even without formal prosecution history estoppel.

3. Defense of "Lack of Effect" (作用効果不奏功の抗弁 - sayō kōka fusōkō no kōben) / Reverse Doctrine of Equivalents

Conversely, situations can arise where an accused product literally meets all claim elements, but it does not achieve the object or effects of the patented invention as described in the specification, or it operates on a fundamentally different principle. While Japan does not have a formally codified "reverse doctrine of equivalents" like that discussed in U.S. jurisprudence, the underlying principle of ensuring that patent protection aligns with the actual inventive contribution is relevant.
If an accused product, despite literal correspondence, fails to deliver the promised utility or works in a radically different way, it can be argued that it does not truly appropriate the patented invention. Some Japanese court decisions have considered such "lack of effect" as a defense, suggesting that extending patent rights to cover such an embodiment would be an overreach beyond the invention's substantial value. Alternatively, if the invention as claimed proves incapable of producing the described effect, this might point to an issue of patentability (e.g., lack of utility or enablement) that could be raised in an invalidity defense.

Comparing with the U.S. Doctrine of Equivalents

The U.S. Doctrine of Equivalents, famously articulated in cases like Graver Tank & Mfg. Co. v. Linde Air Products Co. (1950) and further refined in Warner-Jenkinson Co. v. Hilton Davis Chemical Co. (1997), also seeks to prevent "fraud on a patent." Key U.S. tests include the "function-way-result" test and the "insubstantial differences" test. U.S. law also recognizes significant limitations, such as prosecution history estoppel (with a presumption of surrender for narrowing amendments related to patentability) and the ensnarement doctrine (equivalents cannot extend to cover the prior art).

Key Differences and Similarities:

• Structured Test: Japan's five-part test provides a more structured framework compared to the somewhat more holistic U.S. approach, though both address similar underlying concepts.
• Time of Assessment for Ease of Substitution: Japan uses the "time of infringement," while the U.S. assesses foreseeability of equivalents at the "time of amendment" for estoppel purposes and insubstantial differences generally at the time of infringement.
• Essential Element: While the U.S. "all elements rule" requires that an equivalent be found for each claimed element or its substantial equivalent, Japan's "non-essential part" inquiry focuses on the core inventive concept.
• Ensnarement/Hypothetical Claim: Both jurisdictions prevent the doctrine from encompassing prior art, though the articulation (Japan's fourth requirement) differs slightly from the U.S. ensnarement doctrine.
• Prosecution History Estoppel: Both systems have robust estoppel doctrines, but the presumptions and the scope of the bar (e.g., Japan's flexible bar) can have different applications.

Conclusion

The Doctrine of Equivalents plays a vital role in the Japanese patent system by providing a necessary counterbalance to the principle of literal claim interpretation. It ensures that patentees are not unfairly deprived of protection due to trivial or obvious substitutions by infringers. However, its application is carefully circumscribed by the five requirements established by the Supreme Court, aiming to balance the patentee's rights with the public's need for legal certainty. For businesses operating in Japan, a thorough understanding of these requirements, and how they might compare or contrast with those in other key jurisdictions like the U.S., is essential for effective patent strategy, whether in drafting claims, assessing infringement risk, or engaging in patent litigation.