Provided are a novel therapeutic agent and therapeutic method for prostatic cancers. More specifically, a prostatic cancer therapeutic/prophylactic agent having a viral envelope vector, particularly a Sendai viral envelope vector, as an active ingredient, the therapeutic/prophylactic agent which is an apoptosis induction promoter, the therapeutic/prophylactic agent used for prostatic cancers whose androgen susceptibility has been partially or completely reduced, and a melanoma therapeutic/prophylactic agent containing a Sendai viral envelope vector as the only active ingredient, and the like are provided.