A 74-year-old woman presented with a pulsatile mass and pain in the right mid-thigh. Angiography demonstrated a ruptured solitary aneurysm at the superficial femoral artery (SFA), 45mm in diameter. The operation consisted of replacement of the aneurysm with a saphenous vein bypass grafting to the SFA. The pathological examination of the resected arterial wall revealed degenerative changes due to cystic medial necrosis. In the literature, there has been no report, as far as we know, on true aneurysm of the superficial femoral artery caused by cystic medial necrosis. Therefore, it is considered that our case is a very rare presentation of aneurysm with this particular etiology which occurred in the peripheral artery of a lower extremity.