A 43-year-old man was admitted to our hospital for rehabilitation of higher brain dysfunction caused by traumatic brain injury. He had undergone an emergency operation for acute epidural hematoma after a fall. He had impaired cognitive function with regard to memory, attention, and executive function. However, he also exhibited facial features such as frontal baldness and hatched face. Neurological examination showed mild distal muscle weakness in the extremities, gait disturbance, and grip and percussion myotonia. Needle electromyography showed myotonic discharges. Brain magnetic resonance imaging did not show any intra-axial abnormalities. Myotonic dystrophy (DM) was therefore diagnosed. Rehabilitation resulted in improved gait stability, but the patient's cognitive function did not improve. Severity of illness in DM patients varies from a floppy infant to a lack of awareness of the disease throughout one's lifetime. Therefore, DM patients might not be properly diagnosed because of the mildness of their clinical symptoms. It is important for non-neurologists to be aware of the possible occurrence of this disease.