A 57-year-old female was admitted to our hospital because of headache, nausea, and vomiting. Head CT scan demonstrated subarachnoid hemorrhage. Cerebral angiography showed the absence of the right internal carotid artery, and skull base CT of the bone window level revealed the absence of the right carotid canal. The right middle cerebral artery (MCA) and anterior cerebral artery (ACA) were opacified from the left internal carotid artery. The right A1 portion was hypoplastic and the distal portion of the right M1 portion was replaced by several minute complicated anastomotic vessels connected to the right M2 portion. The right MCA territory was mainly supplied by collateral flow from the right ACA and the right posterior cerebral artery via the leptomeningeal anastomosis. These was neither aneurysm nor arteriovenous malformation. The second angiography, 1 week after the initial angiography, showed the same hemodynamic pattern and aneurysms were not found. We diagnosed the patient as agenesis of the right internal carotid artery and the etiology of subarachnoid hemorrhage was suspected to be a rupture of the anastomotic vessels between the right M1 and M2. She was discharged on the 21st hospital day without any neurological deficit.