On February 24, 1998 the Supreme Court rendered a decision which gave a set of guidelines to determine whether patent infringement under the doctrine of equivalents should be found. Applying the guidelines, the Court reversed a controversial Tokyo High Court decision that had found patent infringement under the doctrine.

After confirming the generally-accepted principle that no infringement should be found if there is an element that is recited in the claim, but is not found in the accused product, the Court further stated as follows:

Even if an element found in the accused product is different from the one recited in the claim, the accused product is equivalent to the claimed invention and within the scope of protection of the patent when all the following conditions are met:

(1) the element does not relate to the core concept of the claimed invention;

(2) replacement of the recited element with the one found in the accused product achieves the purpose of the invention; that is, they perform the same effect;

(3) those skilled in the art would have no difficulty in conceiving the replacement at the time of manufacture of the accused product;

(4) the accused product would not have been the same as, or obvious from, the prior art at the time of filing of the patent application; and

(5) there is no special circumstance that indicates or implies that the accused product was intentionally excluded from the scope of the claim during the prosecution of the patent application.

The Supreme Court reversed the Tokyo High Court decision on the ground that the lower court had failed to consider condition (4) above, that is, whether the accused product was obvious from the prior art. The case was remanded to the Tokyo High Court.