A 71-year-old man was admitted to the department of general surgery at our hospital due to constipation. A large bowel endoscopic examination revealed a stenosis of the rectum near the anus. The pathological diagnosis of the biopsy was poorly differentiated adenocarcinoma. After a computed tomography/magnetic resonance imaging examination, rectal cancer infiltrating the prostate was the diagnosis. External beam radiation therapy and chemotherapy were performed. After those neoadjuvant therapies, an abdominoperineal resection of the rectum (Miles) and a retropubic radical prostatectomy were performed. The final pathological diagnosis was prostate cancer infiltrating the rectum. Prostate cancer infiltrating the rectum is rare because of the Denonvillier's fascia barrier. However, it is difficult to distinguish prostate cancer infiltrating the rectum from rectal cancer infiltrating the prostate. Thus, when we see rectal cancer infiltrating the prostate, prostate cancer infiltrating the rectum should be suspected, serum prostate specific antigen (PSA) level should be determined, and PSA immunostaining should be performed.