An unauthorization determination system includes a memory device that is mounted in a vehicle and includes a predetermined non-access area, a connection device mounted in the vehicle and configured to be connected to the external device in a wired or wireless manner, and an ECU configured to, in a case where access to the non-access area is requested from the external device or in a case where the non-access area is accessed by the external device, determine that the access from the external device is unauthorized.