A 54-year-old man presented with unconsciousness. Computed tomography revealed acute subdural hematoma. Emergency evacuation of hematoma was performed showing any excessive tendency to bleed or difficulty to stop bleeding during the operation. However transfusion of fresh frozen plasma was needed to stop continuous bleeding from the surgical wound after the operation. The patient underwent craniotomy again 18 days after the operation because he suffered hemorrhagic infarction and recurrence of acute subdural hematoma. After the second operation, a coagulability examination revealed that his activated partial thromboplastin test was prolonged (74.5 seconds) and his plasma factor VIII level was 20% of normal, so he was diagnosed as having mild hemophilia A. Cranioplasty was accomplished with replacement therapy, and he was discharged with mild recent memory disturbance and homonymous hemianopsia.