A 77-year-old man was seen at our hospital with the chief complaint of pollakisuria. Magnetic resonance imaging (MRI) showed a 25 mm cystic tumor with solid components behind the prostate. A transrectal biopsy for the prostate showed no evidence of malignancy. Two years later, he complained of weak urinary stream, and the MRI diagnosis demonstrated an increase of the tumor size to 67 mm. Since prostatic sarcoma was diagnosed by the transrectal biopsy for the prostate, a tumor resection and prostatectomy were performed. At 19 months after the operation, there was no evidence of metastasis or recurrence, and he has had no dysuria. The final pathological diagnosis was of a prostatic STUMP.