Anti-theft system using current position information

This invention is an anti-theft system (1,2,3) that is capable of preventing theft of a vehicle-mounted device, and improving the convenience to a user. This invention creates current-position information for a vehicle that indicates the current position (122,...,126) of the vehicle, and projects the created current-position information for the vehicle on map information that was stored beforehand, and when the current-position information for the vehicle matches specified position information (120,121) that is created based on arbitrary conditions set by the user, it makes it impossible to detect a vehicle-mounted device that can seen from outside the vehicle. This can be achieved by concealing the device by hiding, reducing its volume or its brightness. Independent claims are also included for an anti-theft system starting up an anti-theft device only if there is a match between the current-position information and the specified position.