A76-year-old man was transferred to our hospital for evaluation of continuous abdominal pain with systemic inflammation. Clinical examinations suggested the pressure of an adynamic ileus due to diverculitis and an infrarenal abdominal aortic aneurysm. Thirty-eight millimeter in diameter despite the commencement of antibiotic therapy, the symptoms progressively worsened and repeated CT scan demonstrated a rupture of the aneurysm without any enlargement in diameter. An emergency operation was done, and the patient was discharged on the 77th postoperative day. Blood culture and histological findings disclosed an infected aortic aneurysm due to Listeria monocytogenes. The possibility of aortic infection should be considered in patients showing a sudden rupture of an aortic aneurysm.