The patient, male and 70 years of age, was suspected of having a ruptured abdominal aortic aneurysm during the laparotomy, and referred to our department. After admission it was found that a ruptured abdominal aortic aneurysm complicated with dissecting aneurysm. Urgent surgical intervention should have been scheduled but an initial conservative control was necessary because of impaired hepato-renal function and hemorrhagic gastroduodenal ulcer. Meanwhile, pleural effusion, edema in lower extremities and abdominal continuous vascular murmur appeared, and cardiac failure symptoms gradually aggravated. The formation of arteriovenous fistula was suspected. Aortography revealed the DeBakey IIIb type dissecting aortic aneurysm complicating an abdominal aneurysmal rupture with the fistula formation to inferior caval vein. Neither enlargement nor progress of a thoracic aortic aneurysm were observed by CT scanning examination and the closure of fistula and Y grafting were performed successfully. So far, only one case of a ruptured abdominal aortic aneurysm forming the aorto-caval fistula and complicating dissecting aortic aneurysm was reported in Japan, and, to the best of our knowledge, this is the first case treated successfully.