We report a 58-year-old woman with adult onset Alexander disease. At the age of 54 she noticed numbness in bilateral legs and at 57 she developed left sided spastic gait. Her walking difficulty was gradually worsened and followed by the development of weakness in left arm, dysarthria and dysphagia. Her mother and elder brother also had similar clinical presentations which suggested an autosomal dominant neurological disorder. With MRI findings showing localized atrophy of medulla oblongata and upper cervical cord with hyperintensities on T2-weighted image, diagnosis of adult onset Alexander disease was made. We performed genetic analysis and found novel variant (S398F) in the glial fibrillary acidic protein gene. In case of slowly progressive myelopathy with bulbar palsy of unknown origin, especially those with atrophy limited to medulla oblongata and upper cervical cord, adult onset Alexander disease should be taken into consideration.