The patient was a 70-year-old male with a chief complaint of chest pain. Coronary angiography revealed a saccular aneurysm of 11 mm in diameter at the furcation of the left main coronary artery. Triple vessel disease was also seen. At surgery, it was judged that the risk to rupture of the aneurysm was high because the aneurysmal wall was very thin. Therefore, the blood flow into the aneurysm was blocked, and coronary artery bypass grafting was performed. In a pathological study on the aneurysmal wall, no atherosclerotic and no inflammatory changes were found, but acidic mucopolysaccharides were detected in the tunica media. Coronary aneurysms with coronary stenosis in the elderly have been reported to be arteriosclerotic without exception. Here we present this rare case of a left main coronary artery aneurysm with coronary stenosis in an old-aged patient, considered to be due to metabolic abnormalities in the smooth muscle cells.