To 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, an anti-theft apparatus according to the present invention comprises an immobilizer provided in the vehicle, and designed to control a gear shifter not to allow the gear shifter to be shifted from a neutral state or a parking state when the person is identified as the unregistered user under the condition that the vehicle is in the stopped state.