The Japanese court decision on May 16, 2024 (the “DABUS Invention Case”), in which the applicability of AI as an “inventor” was disputed, provided an important opportunity to question how Japanese patent law should respond to the development of AI. The day is not far off when “AI fully autonomous inventions” will be realized, which will not require any human creative involvement. The “inventor” as defined in patent law is interpreted to be limited to a “natural person,” but with the future evolution of AI, how to treat “AI fully autonomous inventions” will be a major issue. The legal issues concerning “AI fully autonomous inventions” are discussed by comparing the decision of the German Federal Court of Justice in the “DABUS Invention Case” on June 11, 2024 with court decisions in Japan. Under the current Japanese patent law, “AI fully autonomous inventions” may cause the problem of absence of “inventor” or presumption. Therefore, we believe that there is a limit to the solution of this problem by the theory of interpretation, and that it is necessary to consider the problem by the legislation.