A 75-year-old woman complaining of sudden lower abdominal pain, fever, and bloody stool was admitted to the hospital. Abdominal CT scan showed retroperitoneal emphysema ranging from the retrorectal space to the posterior region of the kidney, and a stool-like shadow around the rectum. It suggested rectal ulcer and conservative management was started since the general condition was not critical and abdominal pain was localized. Colonoscopy revealed a solitary ulcerative lesion on the posterior wall of rectosigmoid colon, which was considered to be perforating origin. We report a case of perforating rectal ulcer with retroperitoneal emphysema successfully treated conservatively.