We report a case involving the development of a delayed acute subdural hematoma (ASDH) after trauma, with the absence of any abnormal radiological and clinical findings at initial examination. A 54-year-old male visited the emergency department after a minor trauma. The patient only complained of mild headache after head injury. He presented no abnormal findings on neurological examination, and brain computed tomography (CT) did not show any intracranial lesion or skull fractures. However, he developed seizure with disorientation eight hours after trauma, and ASDH with midline shift was found during a follow-up CT. He recovered without neurological deficits after immediate primary care and admission to the neurosurgery department. On serial follow-up CT images, a gradually increasing mass effect of hematoma was detected, and removed by craniotomy. The patient recovered without neurologic deficits.