We report a ruptured anterior communicating artery (ACoA) aneurysm in a patient whose anterior cerebral artery (ACA) arose from the left C3 portion and ascended between the optic nerves. A 50 year-old woman was admitted to our department with complaints of headache and vomiting. CT scan showed thick subarachnoid hemorrhage. Left carotid angiography revealed a saccular aneurysm of the anterior communicating artery and an abnormal vessel which arose from the left C3 portion and terminated in the territory of the ACoA. Surgery was performed about eight hours after subarachnoid hemorrhage. The abnormal vessel was identified as it ascended between the optic nerves. There are only 25 reported cases of this abnormality including our own case. The right carotid artery was the most frequent origin of the abnormal artery (18 cases). 13 aneurysms were associated in 11 cases, in which seven were located in the anterior communicating artery. It was the most frequent site of the aneurysm. There were various explanations for this anomalous ACA. Because of the angiographical and operative findings, we prefer to use "inter-optic course of ACA" as the medical terminology better suited to describe this condition.