An infected aneurysm (IA) is a relatively rare but complex and life-threatening disease. We report a 78-year-old man with an IA in the common iliac artery (CIA) due to Clostridium perfringens. An initial computed tomography (CT) revealed an air pocket in the left CIA, and a pseudoaneurysm was seen on the CT taken the next day, in the area where the air pocket was initially observed. Due to the patient's high surgical risk, emergent endovascular aneurysm repair (EVAR) was performed. No indolent infection was found 1.5 years after the surgery. Because of its high risk of expansion and rupture, accurate diagnosis and immediate treatment is required for managing IAs. The case emphasizes that air density in an arterial wall could be an early radiologic feature of an IA, and EVAR could be a treatment option for IA.