According to Japanese Patent Act, the Japanese Patent Office, administrative organization, was authorized to decide validation of patent. However, Supreme Court of Japan held that a court is able to decide the invalidation of patent in 11th April, 2000, which caused the reform of Japanese Patent Act in June 2004. Reformed Patent Act established the article 104-3 and makes it for a court to decide the patentability where there are grounds for a patent invalidation. Through this amendment to the Patent Act, the legislative system to decide the patent validation has been reorganized and furthermore alleged infringer is allowed to argue against the patent validation by making use of infringement litigation procedure through defenses against patent invalidation as well as invalidation trial procedure for to file a request for a trial for patent invalidation to the Japanese Patent Office. That is to say, the article 104-3 was established in the Japanese Patent Act in the wake of Kilby, and thus a court, which is judicial authority, not administrative disposition agency is also able to decide the patent validation. Thus this article discuss how a court, the authority of which only patent infringement cases fell under, has been authorized to arbitrate cases about the patent validation and the decision of the patent validation in a court.