A 75-year-old female presented with subarachnoid hemorrhage. Angiography revealed a partial duplication (fenestration) in the proximal $A_1$ segment and a ruptured aneurysm at the distal end of $A_1$ fenestration. This congenital anomaly accompanying an aneurysm was associated with duplicated ipsilateral middle cerebral artery (MCA). Congenital defect of the arterial wall and hemodynamic factors at the fenestrated $A_1$ are considered to play a significant role in the development of this aneurysm. The present case is peculiar because not only the ruptured $A_1$ aneurysm was related with the anterior and middle cerebral artery duplication but also the location of $A_1$ fenestration and the origin of $A_1$ aneurysm in a fenestration are quite unusual.