We encountered a case of nontraumatic arterial dissection of the anterior cerebral artery (ACA) which exhibited cerebral infarction and subarachnoid hemorrhage (SAH) simultaneously, and whose symptoms were improved by conservative treatment. A 55-year-old female presenting with headache and weakness in her left leg was admitted to our hospital. CT scans on admission revealed SAH in the interhemispheric fissure and surface of the right frontal lobe, but CT scans at 3 days after onset demonstrated cerebral infarction in the medial part of the right frontal lobe. Cerebral angiography on day 6 disclosed an aneurysmal dilatation and narrowing at the right A2-A3 segment. We continued conservative therapy including blood pressure control, since there was no symptomatic deterioration. The aneurysmal dilatation disappeared and the weakness of the left leg also improved. This case indicates that conservative treatment could be an option for the management of nontraumatic arterial dissection of the ACA with simultaneous cerebral infarction and SAH.