A 70-year-old woman presented with headaches and recurrent stroke symptoms. During five years, the patient has been treated for cerebral infarction associated with severe atherosclerotic stenosis of the internal carotid artery. Three-year follow-up magnetic resonance angiography showed a tiny de novo aneurysm arising from the distal part of atherosclerotic internal carotid artery. And 5-year follow-up three-dimensional CT angiogram demonstrated a definite aneurysm enlargement as large as requiring treatment. During dissection of aneurysm, the oculomotor nerve was found to be penetrated with the growing de novo aneurysm. The authors report a case of a de novo aneurysm, which resulted from atherosclerotic stenosis of the internal carotid artery at the supraclinoid portion, that was found to be penetrating the oculomotor nerve with no ocular palsy.