A 59-year-old woman visited our institute with the chief complaint of dizziness which persisted whenever she tried to focus on objects. She had not experienced apparent double vision and had no history of intracranial bleeding. Neurological examination revealed no abnormality except for exotropia at the mid-position and at upper gaze. Cerebral angiography revealed that the intracranial portion of the left internal carotid artery ran more horizontally and also identified an unruptured left internal carotid-anterior choroidal artery (IC-AChA) aneurysm of 3.0 mm diameter. The aneurysm at the origin of the AChA was confirmed during surgery. The proximal lateral wall of the aneurysm was in contact with the oculomotor nerve. This contact was released after complete obliteration of the aneurysm. The exotropia resolved 3 months later. Oculomotor nerve palsy usually indicates the presence of internal carotid-posterior communicating artery (IC-PcomA) aneurysm. Since sacrifice of the AChA will result in severe neurological deficits, accurate neuroimaging information is needed prior to the operation. Conventional angiography and/or three-dimensional computed tomography angiography should be performed to ascertain whether the aneurysm is an IC-PcomA or IC-AChA aneurysm, even if some neurosurgeons insist that conventional angiography is not always needed before surgery for an unruptured aneurysm.