Isolated oculomotor nerve palsy is well known as a symptom of microvascular infarction and intracranial aneurysm, but unilateral oculomotor nerve palsy as an initial manifestation of chronic subdural hematoma (CSDH) is a rare clinical condition. We report a rare case of an 84-year-old woman with bilateral CSDH who presented with unilateral oculomotor nerve palsy as the initial symptom. The patient, who had a medical history of minor head injury 3 weeks prior, presented with left ptosis, diplopia, and vomiting. She had taken an antiplatelet drug for lacunar cerebral infarction. Computed tomography (CT) of the head showed bilateral CSDH with a slight midline shift to the left side. She underwent an urgent evacuation through bilateral frontal burr holes. Magnetic resonance angiography (MRA) after evacuation revealed no intracranial aneurysms, but constructive interference in steady-state (CISS) magnetic resonance imaging (MRI) revealed that the left posterior cerebral artery (PCA) ran much more anteriorly and inferiorly compared with the right PCA and the left oculomotor nerve passed very closely between the left PCA and the left superior cerebellar artery (SCA). There is the possibility that the strong compression to the left uncus, the left PCA, and the left SCA due to the bilateral CSDH resulted in left oculomotor nerve palsy with an initial manifestation without unconsciousness. Unilateral oculomotor nerve palsy as an initial presentation caused by bilateral CSDH without unconsciousness is a rare clinical condition, but this situation is very important as a differential diagnosis of unilateral oculomotor nerve palsy.