To suppress theft of a vehicle through the replacement of the whole of an immobilizer.

An anti-theft device which is mounted in a vehicle and which communicates with a communication device for executing control of the vehicle, is equipped with: an actuation determination means for determining whether the actuation of the vehicle is permitted; and a communication means for transmitting a signal for permitting the actuation to the communication device by radio when the actuation is permitted by the actuation determination means. The communication means is not allowed to communicate with the communication device when the anti-theft device is mounted in a position other than a position to be mounted.