We report a case of successful treatment of a mycotic aneurysm of the thoracic aorta. A 65-year-old man with a dissecting aneurysm presented with urinary tract infection. He had a history of severe liver cirrhosis. Two weeks after admission, he had a high-grade fever and enhanced computed tomography (CT) demonstrated acute expansion of the distal aortic arch aneurysm. Because of the acute aneurysm expansion and elevated inflammatory response, we suspected a mycotic aortic aneurysm with possible impending rupture. Since conventional open chest surgery was considered to carry a high operative risk, the patient was managed with a combination of emergency endovascular treatment and antibiotic chemotherapy. Extended-spectrum beta-lactamase-producing Escherichia coli were identified from blood culture before treatment. After strict antibiotic therapy, the postoperative course was uneventful and the patient remained well 12 months later.