A 47-year-old man was admitted to the emergency department due to a sudden onset of headache during exercise. Brain computed tomography (CT) and CT angiography showed a subarachnoid hemorrhage (SAH) with a small sac in the left posterior communicating (pcom) artery and total obstruction in the proximal portion of the right middle cerebral artery (MCA). Catheter angiography revealed a protruding lesion in the left p-com artery and a total obstruction in the right MCA. It was difficult to differentiate the aneurysm from the occlusion of the pcom artery. We planned to confirm whether or not the protruded lesion was an aneurysm. Intraoperatively, the aneurysmal opacification seen on the preoperative catheter angiography proved it to be the occluded p-com artery filled with thrombus. The possibility of a vascular stump should be considered when diagnosing a protruding vascular lesion at the p-com artery.