A navigation device includes a base unit 10 which is mounted to a body of a vehicle, a front panel unit 14 which is detachably mounted to and connected to the base unit 10, the front panel unit having a navigation function of displaying map data on a front panel and displaying a location of the vehicle on the map data, and an authentication unit 30 which performs authentication to confirm that the base unit 10 and the front panel unit 14 have been connected to each other and, when confirmed, permits operation of an electrical control unit of the vehicle. The navigation device can be used as an antitheft device.