A 33-year-old man with Wernicke' encephalopathy due to diet and malnutrition is reported. The patient showed upbeating nystagmus and lateral gaze nystagmus with dizziness and ataxia. Caloric testing demonstrated bilateral canal paresis for horizontal eye movement, but bilateral upbeating nystagmus for vertical eye movement. Optokinetic responses were severely disturbed. Neuro-otological examinations indicated lesions of the cerebellum and brainstem including bilateral vestibular nuclei. Various causes such as alcoholism, hyperemesis gravidarum, hyperalimentation without thiamine, and gastric plication, have been reported in Wernicke' encephalopathy however, this case was considered due to diet and malnutrition, which has rarely been noted. It is necessary to pay close attention to information from the patient regarding nutrition and the pattern of nystagmus in order to diagnose Wernicke' encephalopathy.