A 70 year-old male was seen at the hospital with the chief complaints of frequent miction and incomplete urinary retention. A hen's egg-sized firm mass was palpable in anterior wall of rectum by digital rectal examination. Intravenous urography showed severe bilateral hydronephroureter. Transrectal ultrasound, CT scan and MRI revealed a mass with 5 cm in diameter between prostate and rectum, and the margin of them were unclear. On needle biopsy of the tumor, leiomyosarcoma of the prostate was suspected. We performed radical cytectomy and created continent urinary reserver. Because the tumor and rectum could not be lysed, part of the rectum was resected. Histological examination showed gastrointestinal stromal tumor (GIST) of rectum. GIST of rectum is a rare entity, and in case of contact with the prostate, it is difficult to differentiate from leiomyosarcoma of prostate.