A 77-year-old woman had suffered from memory disturbance and disorientation for two years before she was admitted to the hospital because of confusion. Her score on the Hasegawa dementia scale revised (HDS-R) was 12 points at the time of the first medical examination. No other abnormalities could be found except for a blood glucose concentration of 34 mg/dl. A Magnetic Resonance Image (MRI) of the brain showed some small lacunae on both sides in the frontal white matter and basal ganglia. After hospitalization, glucose was administered and the blood glucose concentration increased, but the dementia-like symptoms did not resolve. She was discharged because the symptoms were too difficult to control in the general hospital. Although dementia-like symptoms were present even after discharge, they did not necessarily appear during fasting. Six months later she was rehospitalized. The insulin-blood sugar ratio was at least 0.3 and abdominal echogram showed a 1-cm tumor at the tail of pancreas. The pancreas tail was removed and the tumor cells were reacted with antiinsulin-antibodies. One month after the operation, the dementia-like symptoms had resolved. The HDS-R score was improved to 27 points (normal range) 40 days after the operation. The amount of the slow waves in the electroencephalogram decreased 5 months after the operation. The dementia-like symptoms observed in this case could be regarded as the Durchgangssyndrom of Wieck. This syndrome is observed transiently at the time of recovery of deterioration of disturbances of consciousness. But it is treatable. This patient was an interesting case that showed Durchgangssyndrom mimiking dementia associated with insulinoma.