## Facts

- THK K.K. owned a patent for an “endlessly sliding ball spline shaft bearing,” a linear motion device that recirculates balls between load-bearing and non-load-bearing grooves to permit continuous sliding while transmitting torque.
- The asserted claim described, among other elements, an outer cylinder with alternating axial ball-guiding grooves (including differences in groove depth) and related structures enabling ball circulation.
- Tsubakimoto Seiko Co. manufactured and sold ball spline shaft bearings that met most claim elements but differed in certain structural aspects, including groove configuration and related features affecting ball circulation.
- THK sued for patent infringement and damages, asserting literal infringement or infringement under an equivalents theory.
- The Tokyo High Court held that the accused products infringed under an equivalents-type approach because they were substantially the same as the patented invention and achieved the same problem-solving result despite structural differences.
- Tsubakimoto appealed to the Supreme Court of Japan.

## Issues

1. Whether Japanese patent law permits infringement findings when an accused product does not literally satisfy a claim element (doctrine of equivalents).
2. If so, what conditions must be satisfied to treat a non-literal variation as an equivalent without improperly expanding claim scope.

## Decision

- The Supreme Court reversed the Tokyo High Court and remanded.
- The Court recognized that infringement may be found even without literal satisfaction of claim language, but only under strict conditions.
- The Court held that the High Court’s “substantially the same” reasoning was legally insufficient because it lacked a structured framework and did not adequately address limits based on claim scope, prior art, and prosecution conduct.
- The Court articulated a five-part doctrine-of-equivalents test and directed reconsideration under that test.

## Legal Principles

- A claim element not literally met may be treated as satisfied by an equivalent only if:
  - the differing portion is not an essential element of the patented invention;
  - the substituted element performs substantially the same function in substantially the same way and achieves substantially the same result;
  - at the time of the alleged infringement, a person skilled in the art could have readily conceived the substitution using common technical knowledge;
  - the asserted equivalent would not improperly capture prior art or otherwise extend protection to what could not validly have been claimed; and
  - the patentee did not intentionally exclude the variant during prosecution (prosecution history estoppel).
- The doctrine of equivalents serves to prevent easy evasion through minor changes, but it is constrained to preserve predictability based on claim language and to prevent post-grant claim redrafting.

## Conclusion

The Supreme Court of Japan confirmed that non-literal variants can infringe under a doctrine of equivalents, but only when a structured five-part test is satisfied, requiring non-essentiality, functional identity, ready substitutability at the time of infringement, and limits based on prior art and prosecution history; the infringement finding was reversed and remanded for application of these constraints.


Tsubakimoto Seiko Co. v. THK K.K., No. 1994 (O) 1083 (Sup. Ct. Japan Feb. 24, 1998)