The Doctrine of Equivalents in Japan: How Can U.S. Patentees Prove Infringement Beyond Literal Wording?

Japanese Attorney at Law - Bengoshi L.L.

31 May 2025 • 8 min read

In patent law, determining infringement often begins with a straightforward comparison of an accused product or process against the literal language of the patent claims. However, innovators sometimes face situations where an alleged infringer has made minor, insubstantial changes to a claimed invention, technically falling outside the precise wording of the claims but still capturing the invention's core essence and benefits. To address such scenarios and ensure fair protection for patentees, Japanese patent law, like that of many other jurisdictions, recognizes the Doctrine of Equivalents (Kintō-ron). This doctrine allows a court to find infringement even if the accused product or process does not literally replicate every element of the patent claims.

The Genesis of the Modern Doctrine in Japan: The Ball Spline Bearing Case

While the concept of equivalency had been discussed in lower courts, the modern framework for the Doctrine of Equivalents (DoE) in Japan was definitively established by the Supreme Court in its landmark judgment on February 24, 1998, often referred to as the Ball Spline Bearing case (Heisei 6 (o) No. 1083). This decision provided much-needed clarity and established a specific set of conditions that must be met for infringement under DoE to be found. The Court acknowledged that requiring patentees to foresee and explicitly claim every possible future variation of their invention would be an insurmountable burden and that allowing competitors to easily circumvent patents through trivial modifications would stifle innovation and contravene principles of fairness.

The Five Requirements for Infringement under the Doctrine of Equivalents

The Ball Spline Bearing case laid down five crucial requirements that a patentee must satisfy to establish infringement under the Doctrine of Equivalents. The accused product or process, even if it differs from a literally claimed element, will be considered equivalent to the patented invention if all the following conditions are met:

1. The Differing Element is Not an Essential Part of the Patented Invention (非本質的部分 - Hi-honshitsu-teki Bubun)

The first condition is that the element(s) in the accused product or process that differ from the literally claimed elements must not constitute an "essential part" of the patented invention. This means that the core inventive concept or the heart of the invention must remain unchanged despite the difference.

Determining what constitutes an "essential part" involves analyzing the claimed invention as a whole, considering its technical significance, the problem it solves, and its contribution over the prior art. A commonly accepted approach, endorsed by the Intellectual Property (IP) High Court Grand Panel in the Makisakarushitōru (Maxacalcitol) case (Judgment of March 25, 2016), is the "solution principle identity theory" (kaiketsu genri dōitsu setsu). Under this theory, an essential part is a characteristic feature or combination of features that embodies the unique technical idea or principle of the invention not found in the prior art. If the modification in the accused product alters this core inventive principle, DoE will likely not apply.

2. Replaceability: Achieving the Same Object and Effects (置換可能性 - Chikan Kanōsei)

The second requirement is that by replacing the claimed element with the differing element found in the accused product/process, the object (purpose) of the patented invention can still be achieved, and substantially the same functions and effects are produced. This is often analyzed using a "function-way-result" type of test, common in U.S. DoE jurisprudence as well, though not always explicitly termed as such in Japan. The critical inquiry is whether the substituted element performs substantially the same function, in substantially the same way, to achieve substantially the same result as the claimed element.

3. Obviousness of Replacement at the Time of Infringement (置換容易性 - Chikan Yōisei)

The third condition mandates that the interchangeability of the claimed element with the element in the accused product/process must have been easily foreseeable or obvious to a person ordinarily skilled in the art (tōgyōsha) at the time of the infringing activity (e.g., at the time of manufacture or use of the accused product). This does not mean the specific replacing element itself had to be known, but rather that a skilled artisan, understanding the patented invention, could have readily conceived of making such a substitution without undue experimentation and would have expected it to work in a similar fashion. This requirement focuses on the foreseeability of the substitution itself.

4. The Accused Product/Process is Not Obvious from Prior Art (公知技術からの非容易推考性 - Kōchi Gijutsu kara no Hi-yōi Suikōsei)

The fourth requirement acts as a crucial safeguard for the public domain. The accused product or process, even if found to be equivalent under the first three conditions, cannot itself have been part of the prior art (publicly known technology) at the time the patent application was filed, nor could it have been easily conceivable from such prior art by a person skilled in the art at that time. This is often referred to as the "ensnarement" doctrine or a "prior art" defense within the DoE context. Essentially, the Doctrine of Equivalents cannot be used to expand the patent's scope to cover what the patentee could not have validly claimed in the first place due to lack of novelty or inventive step over existing technology.

5. Absence of Special Circumstances (e.g., Prosecution History Estoppel) (特段の事情の不存在 - Tokudan no Jijō no Fu-sonzai)

The fifth and final condition is that there must be no "special circumstances" indicating that the accused product or process should be excluded from the scope of equivalents. The most prominent example of such special circumstances is prosecution history estoppel (kinhangen). If the patent applicant, during the examination process before the JPO, amended the claims or made arguments that clearly and deliberately excluded or surrendered the subject matter now asserted as an equivalent (typically to overcome prior art rejections or other objections), they cannot later recapture that surrendered subject matter through the Doctrine of Equivalents.

The Supreme Court of Japan provided further clarification on this fifth requirement in a subsequent Makisakarushitōru (Maxacalcitol) case judgment on March 24, 2017. The Court indicated that such special circumstances exist if it is objectively apparent from the prosecution history that the applicant was aware of the alternative technology (which is now part of the accused product) and deliberately chose not to include it within the scope of the claims. An example could be where an applicant discloses an alternative in the specification but then specifically limits the claims to exclude that alternative. This underscores the importance of carefully managing statements and amendments made during patent prosecution.

Applying the Five Requirements: Insights from Jurisprudence

The application of these five requirements is a fact-intensive inquiry. The Tokyo District Court judgment on March 23, 2000, in a case concerning a "Raw Laver Foreign Matter Separation and Removal Device" (Nama-nori Ibbutsu Bunri Jokyo Sōchi Jiken), provides an illustration of how these factors might be analyzed (though the patent in that specific case was later invalidated on other grounds). In that case, the accused device differed in the specific way a rotating plate was positioned relative to a stationary part, falling outside literal claim language. The court, finding no literal infringement, proceeded to a DoE analysis:

It found the differing element was non-essential to the core inventive principle of using centrifugal force and a specific clearance for separation.
It found replaceability, as the accused device achieved the same overall object and effects.
The other conditions were also deemed met, leading to a finding of infringement under DoE at the district court level.

This case, among others, demonstrates that Japanese courts will meticulously examine each of the five conditions. The burden of proof for establishing the first three conditions (non-essential part, replaceability, ease of replacement) generally rests with the patentee. The accused infringer typically bears the burden of proving the fourth condition (that the accused product was obvious from prior art) or the fifth condition (the existence of special circumstances like prosecution history estoppel).

Distinguishing Equivalency Scenarios: Later-Developed vs. Pre-Existing Alternatives

An important consideration in DoE analysis is whether the differing element in the accused product represents a technology that was developed after the patent application was filed or one that already existed at that time.

Later-Developed Alternatives: The rationale for DoE is arguably strongest when an infringer uses a technology developed post-filing to replace a claimed element. The patentee could not have reasonably foreseen and claimed such later-developed alternatives. The Ball Spline Bearing Supreme Court itself highlighted that allowing circumvention through such later-developed technologies would be unjust.
Pre-Existing Alternatives: If the alternative technology used in the accused product already existed and was known at the time of the patent filing, the application of DoE can be more complex. If a patentee was aware of such alternatives (or reasonably should have been) but did not include them in the claims, it raises questions about whether this omission was intentional or simply an oversight. The fifth DoE requirement ("absence of special circumstances") becomes particularly pertinent here. If the prosecution history or the specification itself objectively indicates that the applicant considered a known alternative but chose to draft claims that did not cover it, this might constitute a "special circumstance" barring the application of DoE to that alternative. Some legal scholars argue that for pre-existing, known alternatives, the focus should be more on a flexible and purposive interpretation of the literal claim language ("substantial identity" or jisshitsu-teki dōitsu-ron) rather than a broad application of true DoE, to ensure that the public notice function of the claims is not unduly compromised. The 2017 Makisakarushitōru Supreme Court decision, by emphasizing objective indications of intentional exclusion, lends support to careful scrutiny in such situations.

Variations on the theme of non-literal infringement include "incomplete use" (fukanzen riyō) or "worsened inventions" (kaiaku hatsumei). These occur when an accused product omits a relatively minor or less critical element of the patented invention, or replaces an element with an inferior one, still achieving a similar (though perhaps less effective) overall result.

Such scenarios can also be analyzed under the five-factor DoE framework. The key questions would be whether the omitted/altered element was indeed "non-essential" (first requirement) and whether the accused product still achieves the object and substantially the same effects of the patented invention (second requirement), even if the effects are somewhat diminished. If the core inventive principle is still appropriated, infringement might be found. An early example of finding infringement in a case of incomplete use (omission of a minor feature for block toys) was the Osaka District Court judgment of May 17, 1968 (the Block Toy case). However, if the omitted element is deemed essential, or if the resulting product functions in a substantially different way or fails to achieve the core object of the invention, DoE would not apply. For example, the IP High Court, in a judgment on July 12, 2005 (the "Greening Soil Stabilizer case"), acknowledged that incomplete use could be a type of DoE but denied infringement because the omitted element was found to be an essential part of the invention.

Practical Considerations for U.S. Patentees in Japan

For U.S. patentees seeking to enforce their Japanese patents or to understand the potential scope of competitors' patents in Japan, the Doctrine of Equivalents presents both opportunities and challenges:

Thorough Pre-Litigation Analysis: Before asserting infringement under DoE, a detailed analysis against all five requirements is crucial. This involves not only comparing the accused product to the patented invention but also examining the prior art and the patent's prosecution history.
Evidence Gathering: Successfully arguing DoE requires compelling evidence, particularly regarding the non-essential nature of the modified element, the substantial sameness of function and result, and the ease of interchangeability from the perspective of a person skilled in the art.
Impact of Claim Drafting: While DoE provides a potential avenue for capturing non-literal infringements, it is not a substitute for careful and comprehensive claim drafting. Where foreseeable, drafting claims that encompass likely variations can reduce reliance on DoE.
Prosecution History Management: Statements, amendments, and arguments made during patent prosecution before the JPO (and potentially in corresponding foreign applications, if relevant evidence is presented) can significantly impact the availability of DoE. Consistency and caution are advised.
Anticipating Defenses: Be prepared for accused infringers to vigorously challenge each of the five DoE requirements, particularly by arguing that the differing element is essential, that the accused product was obvious from prior art (ensnarement), or that prosecution history estoppel applies.

Conclusion

The Doctrine of Equivalents plays a vital role in the Japanese patent system, providing a mechanism for patentees to assert their rights against infringers who make insubstantial modifications to a patented invention. The five-requirement framework established by the Supreme Court in the Ball Spline Bearing case, and further refined by subsequent decisions like Makisakarushitōru, offers a structured approach to this equitable doctrine. While it offers a broader scope of protection beyond literal claim language, its application is carefully circumscribed to balance the interests of the patentee with the public's need for legal certainty and the freedom to operate outside the legitimate bounds of a patent. For U.S. businesses and patentees, a nuanced understanding of these principles is indispensable for effective patent enforcement and strategic management of intellectual property in Japan.