We report a case of retroperitoneal gas gangrene, which was caused by cecal diverticulitis with perforation. A-57-year-old male was admitted to the Sado General Hospital with the chief complaint of right lateral abdominal pain. Roentogenogram and Computelized Tomography (CT) showed gas accumulation in the retroperitoneal space behind the ascending colon. Based on the clinical, labolatory, and instrumental examination findings gas gangrene was diagnosed. Since urolithiasis or urinary tract infection was suspected to be the cause of the lesion at that time, the patient was transferred to our department immediately. CT scan done on day 3 at our inpatient department provided data suspicious for the cecal perforation into retroperitoneal space due to appendicitis or diverticulitis. We performed an acute drainage of the abscess and intensive care including continuous hemodiafiltration (CHDF), oxygen under high pressure (OHP), and chemotherapy with antibiotics was carried out. However, in spite of the above mentioned measures, the patient's condition deteriorated and he died due to progression of gangrene and multiple organ failure in 23 days. The autopsy revealed that the cause of perforation was cecal diverticulitis. Retroperitoneal gas gangrene is an uncommon entity and has been rarely reported. It is supposed that laparotomy with diagnostic and therapeutic purpose should have been performed in this case.