Insulinoma with hyperproinsulinemia and normal serum insulin level is a rare disease. Because of the neuroglycopenic symptoms, the initial diagnosis tends to be made as epilepsy or as psychosis. A 43-year-old man was admitted to our hospital because of recurrent confusional episodes. Symptoms are intermittent and consist of staring, confusion, amnesia, and bizarre behavior. Vital signs during the episode were normal but the serum glucose level was 27 mg/dl. The serum level of insulin during the episode was lower than normal and those of proinsulin and growth hormone were higher than normal. Solitary pancreatic mass was found by abdominal CT, measuring 15 mm in diameter. Pathologic evaluation showed islet cell tumor. This suggests that the serum level of proinsulin should be checked when insulinoma with neuroglycopenic symptom is suspected.