The Imperial Throne of Japan is succeeded to by male descendants in the male line of Imperial Lineage (Constitution, Article 2, and Imperial House Law, Articles 1 and 2).
When there is no Imperial Family member specified above, the Throne is passed to the member of the Imperial family next nearest in lineage. (Imperial House Law, Article 2).
Upon the demise of the Emperor, the Imperial Heir ascends the Throne (Imperial House Law, Article 4).