The author outlines the general trends of crime and the treatment of criminals in Japan, and trends of mentally disordered offenders in Japanese penal institutions. Further, he explains the treatment of the mentally disordered offenders in Japanese medical prisons. The author analyzes the Hachioji Medical Prison's treatment record, particularly in regards to the reincarceration rate, which markedly decreases when family support and psychiatric treatment are properly present after release. Finally, the author stresses the value of correctional psychiatric treatment in the medical prison; it is expected to play an important role in contributing to the rehabilitation of mentally disordered offenders and "patients difficult to treat," and bringing about more open and innovative community-centered mental health services.