We report a rare case of an unruptured cerebral aneurysm in the left distal middle cerebral artery(MCA). A 76-year-old woman was referred to our department for unruptured cerebral aneurysms detected incidentally by magnetic resonance angiography. A left carotid angiogram revealed a saccular aneurysm arising in an arterial trunk unrelated to the branching zone of the distal MCA. The patient underwent an operation for trapping and excision of the saccular aneurysm in the distal MCA followed by end-to-end anastomosis. Histological analysis revealed an aneurysmally-dilated artery exhibiting thickened fibrous intima with foci of calcification and fragmented internal elastic lamina. Neither dissection nor infection were observed. The histological findings suggested that the aneurysm in this case was associated with arteriosclerotic change. Considering the medical history of the patient, we conclude that aging and hypertension are factors of the development of the cerebral aneurysm in our case, although the aneurysm is located in the distal MCA in which it is considered to low hemodynamic stress. We report this extremely rare case with a literature review.