Doctrine of Equivalents in Japanese Patent Infringement: What Are the Five Requirements?

In patent law, the determination of infringement typically begins with a comparison of the accused product or process against the literal language of the patent claims. If an accused embodiment does not fall squarely within this literal scope, it generally does not infringe. However, patent rights could be easily circumvented if infringers could make trivial, insubstantial modifications to a patented invention while retaining its core functionality and purpose. To address this, many legal systems, including Japan's, recognize the "Doctrine of Equivalents" (DOE), which allows a patentee to establish infringement even when the accused product or process is not identical to the literal claim language.

The Foundation: Literal Infringement and Its Limitations

The technical scope of a patented invention in Japan is, as a rule, determined based on the statements in the patent claims (Article 70, Paragraph 1 of the Patent Act). Thus, literal infringement occurs when an accused product or process embodies every element set forth in a patent claim.

However, strict adherence to literalism can sometimes lead to unfair outcomes. Language itself has limitations in perfectly capturing the essence of a technical creation. Furthermore, technology evolves, and after a patent is filed, new ways to achieve similar results with minor variations might become apparent. If patent protection were confined only to exact literal embodiments, the value of a patent could be significantly undermined by competitors making only colorable changes. The Doctrine of Equivalents aims to provide more robust and effective protection by preventing such "easy copyists" from unjustly profiting from an inventor's contribution.

The Landmark Ball Spline Bearing Case in Japan

While the concept of equivalence had been discussed in Japanese case law previously, the Supreme Court of Japan, in its seminal decision on February 24, 1998 (often referred to as the Ball Spline Bearing case), formally established a comprehensive five-part test for applying the Doctrine of Equivalents. This decision laid down the specific conditions that must be met for a court to find infringement under this doctrine.

The Five Requirements for Equivalent Infringement in Japan

According to the Supreme Court's ruling in the Ball Spline Bearing case, infringement under the Doctrine of Equivalents can be established if the following five requirements are satisfied:

1. The Substituted Element is Not an Essential Part of the Patented Invention (Non-Essential Part)

The first condition stipulates that the difference between the patented claim and the accused product or process must pertain to a non-essential part of the patented invention. An "essential part" is understood to be the core of the technical idea underlying the patented invention—that which is characteristic and central to solving the problem addressed by the prior art. If the element that has been substituted or altered in the accused product is fundamental to the invention's unique contribution, the doctrine will not apply. This requirement ensures that the Doctrine of Equivalents does not improperly extend patent protection to cover technical ideas that are substantially different from what was actually patented. It prevents the doctrine from being used to claim an entirely separate invention that should rightfully undergo its own patent examination.

2. The Object of the Patented Invention Can Still Be Achieved and the Same Effects Are Produced (Interchangeability of Function and Effect)

The second requirement is that the accused product or process, despite the substitution, must still be capable of achieving the object of the patented invention and must produce the same operational effects. If the substitution results in a significantly different function or a substantially altered outcome or effect, then the accused product cannot be considered an equivalent. This condition focuses on whether the accused embodiment performs substantially the same function in substantially the same way to achieve substantially the same result as the claimed invention.

3. The Substitution Would Have Been Easily Conceivable by a Person Skilled in the Art (Ease of Substitution)

The third criterion is that the substitution of the claimed element with the element found in the accused product or process must have been easily conceivable (or obvious) to a person of ordinary skill in the relevant art at the time of the manufacture of the accused product or the use of the accused process (i.e., at the time of infringement). The rationale here is that if such a substitution was readily apparent to a skilled artisan, then allowing it to escape infringement would be to reward trivial design-arounds. A person skilled in the art is expected to consult the patent claims when developing new technology; if, based on the understanding of the claimed invention, a particular substitution is obvious, then holding such a substitution as an equivalent does not impose an unforeseeable burden on that person.

4. The Accused Product/Process Was Not Publicly Known Technology or Obvious from Prior Art at the Time of Filing (Not Within the Public Domain)

The fourth requirement acts as a safeguard for the public domain. The accused product or process, incorporating the substitution, must not have been technology that was already publicly known at the time the patent application for the invention was filed. Furthermore, it must not have been something that a person skilled in the art could have easily conceived from the publicly known prior art at that filing date. This condition ensures that the Doctrine of Equivalents does not inadvertently expand the scope of the patent to encompass subject matter that was already available to the public or would have been unpatentable for lacking novelty or inventive step at the time the original application was filed. It preserves the principle that patent rights should not cover what is rightfully in the public domain.

5. No Special Circumstances Exist, Such as Intentional Exclusion During Prosecution (Absence of Special Circumstances)

Finally, equivalent infringement will not be found if "special circumstances" exist. The most prominent example of such special circumstances is if the subject matter of the alleged equivalent was intentionally excluded from the scope of the patent claims during the patent prosecution process. This is commonly known as prosecution history estoppel. If an applicant, in response to rejections based on prior art or for other reasons, amends the claims to narrow their scope, or makes arguments that clearly disavow certain subject matter, they generally cannot later recapture that surrendered subject matter through the Doctrine of Equivalents. This principle is rooted in fairness and prevents a patentee from taking contradictory positions before the patent office and the courts. Such intentional exclusion can also arise during post-grant correction proceedings (e.g., a trial for correction).
There can be nuances to this, for example, whether any narrowing amendment automatically triggers estoppel, or if the reasons for the amendment and whether it was made to overcome a specific rejection are relevant factors.

Illustrating the Complexity: The "Magic Tape" (Fastener) Case

Even before the Supreme Court's definitive five-part test, Japanese courts grappled with the concept of equivalence. A notable example involved a dispute over a fastener, commonly known as "Magic Tape" or hook-and-loop fastener (Osaka District Court, April 2, 1969; Osaka High Court, June 26, 1972). This case highlights the complexities involved and how different judicial levels can reach different conclusions.

The patented invention described a fastener made of two flexible supports: one support had numerous "hooks" on its surface, and the other had numerous "loops." When pressed together, the hooks engaged the loops. The accused product, however, used "mushroom-shaped elements" on one support instead of hooks, with the other support still featuring loops.

The Osaka District Court (first instance) found infringement under an equivalent theory. It reasoned that the core inventive concept lay in providing one of the supports with a multitude of loops, which significantly increased the chances of engagement with the elements on the other support, thereby improving the fastener's resistance to separation. From this perspective, the specific shape of the engaging elements (hooks versus mushroom heads) was considered less critical.

However, the Osaka High Court (appellate court) reversed this decision, finding no infringement by equivalence. The High Court placed greater emphasis on the differences in the strength of engagement achieved by the mushroom-shaped elements compared to the claimed hooks, concluding that they did not produce the "same effect" in the same way. The appellate court also appeared to take a different view on the ease of substituting hooks with mushroom-shaped elements, and notably, it assessed this ease of substitution as of the patent's priority date, a different timing standard than the "time of manufacture/use of the accused product" later adopted in the Ball Spline Bearing case.

This fastener case illustrates how the application of equivalence principles, particularly in defining what constitutes an "essential part" or the "same effect," can be highly fact-dependent and subject to differing judicial interpretations.

Effects of a Finding of Equivalent Infringement

When equivalent infringement is established, the legal consequences are the same as those for literal infringement. The infringer may be subject to an injunction to stop the infringing activities and may be liable for damages. While the Doctrine of Equivalents serves the crucial purpose of ensuring effective patent protection, its application must be carefully managed by the courts to avoid unduly broadening patent rights beyond what was fairly disclosed and claimed by the inventor, which could otherwise unfairly impede legitimate competition and innovation.

Conclusion

The Doctrine of Equivalents in Japan, solidified by the Supreme Court's five-requirements test in the Ball Spline Bearing case, provides a vital mechanism for patentees to assert their rights against infringements that fall outside the strict literal wording of their claims but are nonetheless substantially the same as the patented invention. These five conditions—non-essential part, interchangeability of function and effect, ease of substitution, non-existence in the public domain at filing, and absence of prosecution history estoppel—collectively define the boundaries within which this doctrine operates. It represents a careful balance between ensuring patentees receive fair protection for their contributions and maintaining legal certainty and a clear scope for the public and competitors.