A 72-years-old male visited our hospital with a complaint of perineal and voiding pain. An elastic hard mass was palpated by rectal examination in the cranial lesion of prostate. On CT scan and MRI it was located at the site of the right seminal vesicle. Trans-rectal needle biopsy specimen showed leiomyosarcoma. Radical cystoprostatectomy with pelvic lymph node dissection was performed at Hokkaido University Hospital. Microscopic examination of the resected specimen revealed the tumor arose from the right seminal vesicle without involvement of the bladder and the prostate gland. Three months after the operation the tumor recurred at lung and pelvic lymph node. He died 10 month postoperatively. Autopsy finding showed many visceral organs metastases without local recurrence. In the past, nine cases of leiomyosarcoma of seminal vesicle were published in the literature. Including our case, pathological diagnosis by core needle biopsy was attempted in five cases, though accurate diagnosis was made in only two cases. In our case, Immunohistological study by SMA and desmin was useful for histological diagnosis preoperatively.