We report a surgical case of mycotic aneurysm of the aortic arch, the possible cause, and the growth of which were retrospectively determined. A 61-year-old man with diabetes mellitus, free of aneurysm had bacteremia develop that was caused by Staphylococcus aureus. A computed tomographic scan was taken 110 days afterward that revealed an aneurysm in his aortic arch. Total arch replacement was performed, and S. aureus was detected in the aneurysm content. We conjectured that bacteremia caused mycotic aneurysm by day 110. The results of intraoperative microscopic examination also led the authors to decide on a regime for the immediate systemic administration of antibiotics.