A 75-year-old man suffered acute subdural hematoma shortly after trivial head trauma. Thirteen hours after a trivial brow to the occipital region, caused by contact with a mat, he suddenly deteriorated to the level of a Glasgow Coma Scale score of 6. Computed tomography demonstrated an acute subdural hematoma on the left and angiography revealed an aneurysm of the distal middle cerebral artery. An emergent craniotomy disclosed no skull fracture and exposed a thick subdural hematoma with no brain contusions. After evacuation of the hematoma, an aneurysm was found on the distal portion of posterior temporal artery, which was compatible with the angiographical findings. The neck of aneurysm was so fragile that neck clipping could not be successfully performed. Therefore, the aneurysm was extirpated, and the bleeding site coagulated with oxidized cellulose reinforcement. Histological examination of the aneurysm indicated a pseudoaneurysm during the early phase of clot formation. The acute subdural hematoma resulted from rupture of this pseudoaneurysm which was formed shortly after the minor head trauma. Rupture of a pseudoaneurysm caused by trivial trauma might be one of the origins for so-called acute "spontaneous" subdural hematoma.