Challenging Patent Validity in Japanese Infringement Suits: The Interplay of Invalidity Defenses and Amendments

Japanese Attorney at Law - Bengoshi L.L.

• 10 min read

When a patent infringement lawsuit is initiated, the primary focus is often on whether the accused product or process falls within the scope of the patent claims. However, an equally critical aspect of patent litigation in Japan, as in many other jurisdictions, is the ability of the defendant to challenge the very validity of the asserted patent. If a patent is deemed invalid, it cannot be infringed. Japanese patent law provides a specific mechanism for this within infringement proceedings, known as the "invalidity defense." Furthermore, patentees are not without recourse when faced with such challenges; they have avenues to correct or amend their patents to overcome validity defects. This interplay between asserting invalidity and seeking patent correction creates a dynamic and complex legal battleground.

This article explores the invalidity defense in Japanese patent infringement litigation, particularly under Article 104-3 of the Patent Act, and examines its intricate relationship with the patentee's right to make post-grant corrections to their patent.

The Invalidity Defense (無効の抗弁 - mukō no kōben) in Japanese Infringement Litigation

Prior to 2000, Japanese courts were generally reluctant to rule directly on patent validity within infringement proceedings, viewing validity as a matter primarily for the Japan Patent Office (JPO) through separate invalidation trials. However, the Supreme Court decision on April 11, 2000 (often referred to as the Kilin case or "Semiconductor Device" case, Heisei 10 (o) No. 364), marked a significant shift. It held that if a patent clearly possesses grounds for invalidation, enforcing such a patent would constitute an abuse of rights. This paved the way for more active consideration of validity issues by infringement courts.

Subsequently, this principle was codified and expanded with the introduction of Article 104-3 of the Japanese Patent Act (effective 2005). This article explicitly allows a defendant in an infringement suit to argue that the patent right should not be enforced because the patent "should be invalidated by a patent invalidation trial."

Rationale and Effect:
The primary rationales behind Article 104-3 are:

  • To facilitate the early resolution of disputes by allowing validity and infringement to be considered in the same forum.
  • To protect defendants from the consequences of enforcing a patent that is, in substance, invalid, even if a formal JPO invalidation decision has not yet been issued.

It is crucial to understand that if an infringement court accepts an invalidity defense under Article 104-3, the patent is treated as unenforceable between the parties (inter partes) in that specific litigation. This does not invalidate the patent per se in an absolute sense (erga omnes); only a final and binding decision from a JPO invalidation trial (or a court decision upholding such a JPO decision) can achieve that broader effect.

The "Obviousness" (Clarity) Requirement for Invalidity:
The Kilin Supreme Court decision initially required that the grounds for invalidity be "clear" or "obvious" (明白性要件 - meihakusei yōken) for the court to deny enforcement. However, Article 104-3 itself does not explicitly retain this "clear and obvious" standard. The prevailing interpretation and current practice are that an infringement court, when assessing an invalidity defense, will conduct a substantive examination of the invalidity grounds much like the JPO would in an invalidation trial, without imposing a heightened threshold of clarity. The grounds for asserting invalidity under Article 104-3 are generally the same as those available in a JPO invalidation trial under Article 123 of the Patent Act (e.g., lack of novelty, inventive step, enablement, written description support, improper amendments, etc.).

The Patentee's Counter: Correction (Amendment) as a Rejoinder

When a defendant raises an invalidity defense, the patentee is not left without options. They can seek to rectify the alleged defects in their patent by making corrections (訂正 - teisei) to the claims, specification, or drawings. In the context of infringement litigation, this is often asserted as a "rejoinder of correction" (訂正の再抗弁 - teisei no sai-kōben) to counter the invalidity defense.

The procedural avenues for making such corrections include:

  • Requesting a correction within an ongoing invalidation trial at the JPO (Article 134-2 of the Patent Act).
  • If no invalidation trial is pending, filing a separate "trial for correction" (訂正審判 - teisei shinpan) with the JPO (Article 126 of the Patent Act).

The outcome of such JPO correction proceedings can then be introduced and considered in the infringement lawsuit.

Key Issues in the Interplay Between Invalidity Defense and Correction

The interaction between an invalidity defense and a patentee's attempt to correct the patent raises several complex procedural and substantive issues.

A. Necessity of a Formal JPO Correction Request to Assert a Correction Rejoinder

A practical question is whether a patentee must have formally initiated a correction proceeding at the JPO to argue in court that their patent could be corrected to overcome the alleged invalidity.

  • One perspective is that a formal JPO request is necessary because the court cannot adjudicate based on a purely hypothetical correction that has not been officially sought or examined by the JPO.
  • However, a more lenient view suggests that if the manner of correction needed to cure the invalidity is straightforward and its outcome predictable (e.g., deleting an obviously invalid alternative from a claim, leaving a clearly valid scope), the patentee might be able to argue the effect of such a correction in court even without a pending JPO proceeding, or before its conclusion. The viability of this often depends on the clarity of the proposed correction and its impact.

B. Pleading and Proving the Correction Rejoinder

A notable court decision (Tokyo District Court, February 27, 2007, "Articulated Transfer Device" case) helped outline the typical elements a patentee would need to establish when asserting a rejoinder of correction:

  1. Formal Correction Sought: That a correction trial request (or a request for correction within an invalidation trial) has indeed been filed with the JPO concerning the relevant claims.
  2. Compliance with Correction Requirements: That the sought correction meets all the substantive requirements for a valid correction under Article 126 (or Article 134-2). These include limitations on the purpose of correction (e.g., narrowing of claims, correction of errors, clarification of ambiguities) and prohibitions (e.g., no addition of new matter, no substantial enlargement or alteration of claims).
  3. Invalidity Cured: That the correction effectively resolves or cures the specific ground of invalidity raised by the defendant.
  4. Infringement of Corrected Claims: That the defendant's accused product or process falls within the technical scope of the patent claims as corrected.

C. Consequences if the Accused Product Falls Outside the Corrected Claims

A critical scenario arises if an allowable correction (that successfully cures the patent's invalidity) results in a claim scope under which the defendant's product no longer infringes. For example, if the defendant was only practicing a feature of the original claim that has now been deleted or narrowed by the correction to avoid prior art. In such cases, the infringement claim against that defendant will fail. There are two main theoretical pathways to this outcome, differing in their legal reasoning:

  1. Element (4) as an Essential Part of the Patentee's Rejoinder:
    This approach considers the fourth element (infringement of the corrected claim) to be an essential component of the patentee's rejoinder. If the patentee cannot prove that the accused product infringes even the validly corrected claim, their rejoinder fails. As a result, the defendant's original invalidity defense (against the uncorrected claim) effectively succeeds, leading to a finding of non-infringement. This places the onus on the patentee to demonstrate that the asserted patent, in its correctable and valid form, is still infringed.
  2. Element (4) Not Part of the Rejoinder; Non-Infringement Based on Corrected Claim Scope:
    This alternative view argues that if the patentee demonstrates that the patent can be validly corrected to cure the asserted invalidity (i.e., elements 1-3 above are met), then the patent is "not to be invalidated" within the meaning of Article 104-3. The invalidity defense is thus overcome. The separate question of whether the accused product infringes the corrected claims then becomes a standard claim construction and infringement analysis issue. If the defendant's product only practiced the now-excised portion of the original claim (e.g., a part that was in the public domain and was removed by the correction), it simply falls outside the scope of the (now validly) corrected claims. Non-infringement is found based on claim scope, not because the invalidity defense against the original claim succeeded. This view may involve interpreting the corrected claims to exclude known prior art (a concept sometimes referred to as 公知技術除外説 - kōchi gijutsu jogai-setsu or public domain exclusion).

While the ultimate practical result (non-infringement for the defendant practicing only the excised part) is often the same, the legal reasoning, and potentially the allocation of burdens, can differ.

Substantive Requirements for Patent Correction

For a correction (amendment) to be valid, whether sought in a standalone correction trial (Article 126) or within an invalidation trial (Article 134-2), it must meet strict substantive requirements:

  • Permissible Purposes: Corrections are generally limited to:
    • Restriction (narrowing) of the scope of claims.
    • Correction of errors in the description or drawings.
    • Clarification of ambiguous statements.
    • A 2011 amendment also allows correction of claim dependencies when a cited claim is deleted (relevant for maintaining the integrity of claim sets).
  • Prohibition on Adding New Matter (新規事項の追加禁止 - shinki jikō no tsuika kinshi): The correction must not introduce subject matter that was not disclosed in the application as originally filed (Article 126(5)). The test is whether the amendment introduces a new technical matter that a POSITA would not have been able to derive from the totality of the original disclosure. "Excluding claims" (除くクレーム - nozoku kurēmu), which explicitly carve out specific subject matter (e.g., to avoid prior art) from a broader claim, are generally permissible if they do not effectively add new technical information but merely define the boundaries more clearly against what was already known or claimed by others.
  • Prohibition on Substantially Enlarging or Altering Claim Scope: The correction must not, in substance, broaden the scope of the claims or change the invention (Article 126(6)). This largely overlaps with the "restriction" purpose and "no new matter" prohibition.
  • Independent Patentability Requirement (独立特許要件 - dokuritsu tokkyo yōken): For corrections that narrow claims or correct errors, the invention as defined by the corrected claim(s) must itself have been patentable (e.g., novel and non-obvious) as of the original filing date of the application (Article 126(7)). This crucial requirement prevents a patentee from correcting a patent into a form that would not have been allowable in the first place. This ensures that the correction process is not used to obtain protection for unpatentable subject matter.

An improper correction that violates these requirements can itself become a ground for invalidating the entire patent (Article 123(1)(viii)).

Procedural Interplay: Correction Proceedings and Infringement Litigation

The interaction between JPO correction proceedings and ongoing court litigation can be complex. A correction decision by the JPO has retroactive effect to the patent's original filing date (Article 128).

A significant issue historically was the "automatic rescission" principle (当然取消しの法理 - tōzen torikeshi no hōri). If an infringement court was reviewing a JPO decision from an invalidation trial (in an action to rescind that JPO decision), and during that court review, a separate JPO decision in a correction trial became final and binding, which altered the claims that were the subject of the invalidation trial, the court generally could not proceed to rule on the validity of the original, uncorrected claims. The Supreme Court held in such cases (e.g., March 9, 1999, "Large-diameter Square Steel Pipe Manufacturing Method" case) that the original JPO invalidation decision must be rescinded, and the matter remanded to the JPO for reconsideration based on the newly corrected claims. This was based on the idea that the JPO, as the specialist agency, should be the first to assess the validity of the claims in their corrected form.

This created procedural inefficiencies, as court proceedings could be nullified by parallel JPO correction decisions. A series of legislative amendments in 1993, 2003, and 2011 sought to streamline this. The general trend has been to more tightly integrate the correction process into the invalidation trial itself and to limit the availability of separate, standalone correction trials when an invalidation trial (or a court action reviewing its decision) is pending. For example, the 2011 amendments generally prevent a patentee from filing a new correction trial while a court case appealing a JPO invalidation trial decision is ongoing (Article 126(2)). They also introduced a system where, in an invalidation trial, the JPO may issue a preliminary notice of its findings, giving the patentee a specific opportunity to request correction within that same proceeding before a final decision is made (Article 164-2). These changes aim to ensure that validity and any necessary corrections are considered more cohesively.

Standing to Assert the Invalidity Defense

While most grounds for invalidity can be raised by any defendant, a special rule applies if the ground for invalidity is that the patent was obtained by someone not entitled to it, such as through misappropriation of inventorship (冒認出願 - bōnin shutsugan). For this specific ground, only the true inventor or their rightful successor in title has standing to request an invalidation trial at the JPO (Article 123(2), proviso). It is debated whether a similar limitation on standing applies when this specific ground of misappropriation is raised as an invalidity defense under Article 104-3 in an infringement court by a defendant who is not the true inventor.

Res Judicata Effects and Re-litigation Concerns

The finality of court decisions on validity (via Article 104-3) and their relationship with subsequent JPO decisions or correction decisions raises complex issues of res judicata and the possibility of re-litigation. The 2011 amendments to the Patent Act introduced Article 104-4, which aims to limit the circumstances under which a finalized infringement judgment can be reopened for retrial (再審 - saishin) based on a later JPO invalidation or correction decision. The policy is to enhance legal stability and prevent parties from endlessly relitigating issues that were, or could have been, fully addressed in the initial infringement proceeding. The specific conditions under which a retrial might still be possible are detailed and depend on factors such as whether the original judgment was for or against the patentee and the nature of the subsequent JPO action.

Comparison with U.S. Practice

U.S. patent litigation also routinely involves defendants challenging patent validity. The standard of proof for invalidity in the U.S. is "clear and convincing evidence," a higher bar than the "preponderance of the evidence" standard typically used by the USPTO in examination or post-grant proceedings.

Post-grant mechanisms for amending claims in the U.S., such as reissue applications and ex parte reexamination, or more recently, motions to amend in AIA post-grant proceedings (like Inter Partes Review - IPR), have their own distinct rules and procedural interactions with concurrent district court litigation. Unlike the Japanese "automatic rescission" principle, U.S. courts are more likely to stay litigation pending the outcome of USPTO post-grant reviews or allow amendments within certain USPTO proceedings that might then affect the court case. There isn't a direct parallel to the integrated JPO invalidation-correction procedure that directly feeds back into an ongoing appeal of the JPO's own decision in the same manner.

Conclusion

The invalidity defense under Article 104-3 provides a crucial mechanism for accused infringers in Japan to challenge the enforceability of a patent directly within infringement litigation. This, coupled with the patentee's ability to seek corrections to address validity defects, creates a sophisticated and often intricate legal process. Understanding this dynamic interplay—the requirements for raising an invalidity defense, the strict conditions for making valid corrections, and the procedural implications of these actions on the litigation—is essential for both patentees seeking to enforce their rights and defendants seeking to protect themselves from claims based on potentially flawed patents in the Japanese system. The evolving case law and legislative reforms continue to shape this area, emphasizing the need for careful legal strategy and up-to-date knowledge.