A 58-year-old man was seen at our hospital because of rectal pain and difficulty in urinating. On digital examination, a hard indurated mass was palpable. CT revealed a mass which involved rectum, prostate and bladder. Colonoscopy showed annular stricture of rectum near dentate line. Rectal biopsy was found to be inconclusive. We could not discriminate between primary rectal carcinoma and prostate carcinoma locally invasive to the rectum. Total pelvic exenteration was performed. Immunohistochemical staining for protate specific antigen (PSA) led to the diagnosis that the origin of the tumor was poorly differentiated adenocarcinoma of the prostate. Denonvilliers' fascia serves as an effective barrier to the posterior extension of carcinoma of the prostate. Therefore, rectal involvement secondary to prostate cancer is rare. The importance of distinguishing a prostatic neoploasm from other type of neoplasm in the rectum needs emphasis because of distinct differences in the treatment and prognosis. We consider that immunohistochemical examination of rectal biopsy may be necessary to differentiate adenocarcinomas of the prostate from those of the rectum when we encounter an annular stricture of rectum.