PURPOSE: To prevent theft of a vehicle by controlling door lock/unlock when a communication data whose identification data is coincident is received and raising alarm when a communication data is received consecutively for a predetermined time or over.

CONSTITUTION: A control means receives a communication data including an identification data from a transmitter 1 and uses a pushbutton switch to apply door lock/unlock control when a communication data whose identification data is coincident is received. Moreover, a judging means measured the reception consecutive time of the communication data whose identification data is coincident and when the communication data is consecutively received for a predetermined time or over, an alarm means 22 is activated to raise an alarm. Thus, when a thief going to invade into a vehicle 10 is found out by the possessor of the vehicle 10 and the communication data is sent continuously for a prescribed time or over, an alarm is raised and invasion of the thief is prevented.