A 71-year-old man was referred to the University Hospital because of left lumbago and a pulsating mass in his umbilical region. An inflammatory abdominal aortic aneurysm 5cm in diameter and left hydronephrosis were identified by enhanced computed tomography (CT). One month after admission, rapid expansion of the aneurysm with sealed rupture were detected by follow-up enhanced CT. The patient immediately underwent an emergency operation. We confirmed fissure on the posterior aneurysmal wall with a localized hematoma. We replaced the aneurysm with a straight prosthetic graft and the postoperative course was uneventful.