Article 60 of the Japanese Penal Code provides: Two or more person who act jointly in the perpetration of a crime are all principals (Co-principal). Co-principal criminality rests on joint perpetration of crimes basically. In this Context, the matter of so-called conspiratorial co-principal (co-principal criminality based on Conspiracy) needs special consideration. The Precedents of Japanese Supreme Court has supported the doctrine of the Co-principal based on Conspiracy. The recent trend of scholarly opinion has been also to support the precedent after the long discussion. Nevertheless, it is not so clear, to what extent a conspirator is punishable as Co-principal, who participates only in the Conspiracy, not directly in the act of crimes. This essay is intended to introduce the present Discussion about the Co-principal based on Conspiracy in japan.