A 58-year-old man who underwent mitral valve replacement for mitral valve prolapse that was complicated by severe mitral regurgitation was referred to our department due to his sudden onset of drowsiness. Brain computed tomography revealed a hemorrhage in the anterior interhemispheric fissure and left frontal lobe. Cerebral angiography demonstrated a saccular aneurysm at the angular branch of the middle cerebral artery (MCA), but there was no evidence of vascular abnormalities at the hemorrhagic area. End-to-end anastomosis of the angular branch was performed after resection of the friable aneurysm and the perianeurysmal parent artery segment. Follow-up cerebral angiography after three weeks showed an aneurysm in the distal anterior cerebral artery (ACA) in the previously hemorrhagic area. This ruptured aneurysm was successfully clipped. An infectious aneurysm was confirmed on the histopathological examination.