Security system for vehicle navigation system and method of detecting identification code

A vehicle navigation system with a security feature capable of detecting that the system has been removed from the vehicle, which uses only one memory for storing an anti-theft code. The navigation system is provided with a GPS receiver containing a clock outputting a time synchronized with present GPS time information, while receiving electric power from the vehicle battery, and returns the time to an initial preset time when not receiving power from the battery, and detects a present position of the vehicle by means of the present GPS time and position information. A CPU detects whether or not the time of the clock has returned to the initial time, an SRAM stores a preset identification code, and a controller requests input of an identification code when the CPU detects that the time of the clock has returned to the initial time, and compares the input identification code to the identification code stored in the SRAM, and operates in accordance with the result of the comparison.