A 76-year-old male was admitted to our hospital because of melena. However, no remarkable findings of rupture were shown by enhanced CT scan and angiography. On the 9th day of admission, he fell into a state of shock because of sudden massive bloody intestinal discharge. Colonofiberscopic findings revealed a primary arterio-enteric fistula. Therefore, an emergency operation was undertaken. Following aneurysmectomy, colostomy was performed in the descending colon. Right axillo-femoral artery bypass was finally performed as an extra-anatomical bypass to secure the right leg blood flow.