An 82-year-old Japanese man, who presented with a fever and abdominal pain, was admitted to our hospital. According to enhanced computed tomography images, the probable diagnosis was abdominal aortic mycotic aneurysm. Eight sets of blood cultures obtained from the patient were negative. Despite administering treatment with vancomycin and ceftriaxone, the aneurysm progressively enlarged. He underwent open debridement surgery and in situ replacement because of an aneurysmal rupture. Bacteroides fragilis was isolated from the tissue culture of the aortic wall. Metronidazole was administered and discontinued without any infection relapse. When faced with similar cases, rare pathogens should thus be considered as possible causes of mycotic aneurysms.