Small cell carcinoma of the prostate is known to have a poor prognosis. We report a case of a large pelvic tumor with small cell carcinoma, which responded well to docetaxel. A 72-year-old man who was receiving androgen-deprivation therapy for prostatic adenocarcinoma presented with constipation. Although the prostate specific antigen level had decreased, a large pelvic tumor was detected between the prostate and the rectum, which caused bowel obstruction. A biopsy of the pelvic tumor revealed small cell carcinoma of the prostate, and chemotherapy with docetaxel and prednisolone was administered. Five months after the administration of docetaxel, the pelvic tumor disappeared completely. At the time of the last follow up, the response was still maintained.