A 48-year-old man who was diagnosed as undifferentiated malignant tumor by needle biopsy was treated with irradiation and anticancer drugs. Autopsy revealed alveolar rhabdomyosarcoma of the prostate with marked infiltration into periprostatic tissue and distant metastasis. Histological diagnosis was performed by special staining as well as the immunoperoxidase technique with myoglobin. This was the first case of alveolar rhabdomyosarcoma of the prostate in Japan.