A 54-year-old man was admitted to our hospital with complaint of sudden headache. The patient had suffered two episodes of transient headache before admission. Computed tomography (CT) revealed acute subdural hematoma (ASDH) on the right side of the cerebral convexity with bilateral extension along the tentorium cerebelli without signs of subarachnoid hemorrhage (SAH) or intracerebral hemorrhage (ICH). Three-dimensional CT angiography and conventional cerebral angiography revealed a left A1-A2 junction aneurysm. Neck clipping of the aneurysm was performed. The aneurysm extended inferiorly, with the dome embedded in the chiasmatic cistern and tightly adhered to the arachnoid membrane. There was no evidence of hematoma in the subarachnoid space. The patient was discharged without neurological deficit. Ruptured aneurysms resulting in ASDH without SAH or ICH are very rare. Radiological investigation such as three-dimensional CT angiography should be performed to find the causative aneurysm in a patient with ASDH with a history of repeated headaches and without traumatic signs or episodes, and the appropriate treatment should be planned with expediency.