We describe a case of a peripheral ruptured middle cerebral artery(MCA)aneurysm showing repeated morphological changes within a short period of 3 months. A 69-year-old woman was admitted to her primary care hospital with headache. Cranial computed tomography (CT) showed subarachnoid hemorrhage (SAH), but ruptured aneurysm was not confirmed on 4-vessel cerebral angiography. Conservative treatment was provided in the form of pain relief and blood pressure control. However, left internal carotid artery angiography (ICAG) conducted 12 days post-onset showed a peripheral MCA aneurysm, which was enlarged 1 week later. The patient did not tolerate balloon test occlusion, showing neurological deficit. Direct surgery was planned, but angiography on day 30 revealed a reduction in aneurysm size. Medical therapy was therefore continued for 1 month; however, the aneurysm showed repeated enlargements over 3 months. The patient therefore consulted our hospital for surgery, which was performed using a transsylvian approach. As we were unable to rule out pseudoaneurysm, we performed superficial temporal artery-MCA anastomosis as a form of trapping surgery. However, the lesion appeared to likely represent a congenital aneurysm when viewed macroscopically, so we performed neck clipping as a definitive treatment. Navigation and motor-evoked potential monitoring were useful to approach the aneurysm and predict the preservation of motor function. Histological examination revealed a congenital aneurysm with organized thrombus. The postoperative course was uneventful and the patient was discharged 2 weeks after surgery without any neurological deficit.