It is an object of the present invention to provide an anti-theft apparatus which can prevent a vehicle from being stolen or improperly utilized when the registered user is not in the vicinity of the vehicle under the condition that its engine is in an idling state.
The immobilizer includes: user identification data storing means 26 having the user identification data stored therein, the user identification data being indicative of a registered user having permission to utilize the vehicle; identifying means 27 for judging whether or not the person is identical to the registered user on the basis of the comparison between the user identification data received from the identification card 10 and the user identification data stored in the user identification data storing means 26; driving state detecting means 28 for detecting whether the vehicle is in a traveling state, or in a stopped state; and locking and unlocking means 24 for controlling a gear shifter provided in the vehicle on the basis of the judgment of the identifying means 27, the locking and unlocking means 24 being adapted to assume an unlocking state to unlock the gear shifter to allow the gear shifter to be shifted from a neutral state or a parking state when the judgment is made that the person is identified as the unregistered user under the condition that the vehicle is in the stopped state.