We report a rare case of perforation of diverticulum of the sigmoid colon with gas in the portal venous system and retroperitoneal emphysema. A 75-year-old man was seen at the hospital because of abrupt onset of lower abdominal pain and anal bleeding, and was admitted with a suspicion of ischemic colitis on an emergency colonoscopy. Conservative therapy was started, but on the 6th day the abdominal pain was intensified and muscular defense appeared. After a Hartmann operation was conducted, septic shock was lasted and the patient died on the 6th postoperative day. Histologic study of the resected material showed perforation of diverticulum of the sigmoid colon. Generalized peritonitis due to diverticular perforation of the large intestine is relatively common in aged people, but this case which were accompanied by gas in the portal venous system and emphysematous change in the retroperitoneum is rare. Some notes are also presented.