PURPOSE: To prevent the theft of an automobile by operating a mounted alarm unit when an foreign object is inserted to abnormally unlock a door lock mechanism and operating the abnormality warning means of a receiver carried by a driver via the radio wave.

CONSTITUTION: A sensor 41 detecting that a foreign object such as a plate or a bar is inserted into a door lock mechanism is provided on it, and its output signal is inputted to a CPU 9. An alarm unit driving means 12 is constituted of a remote control transmitter 10 carried by a driver and a receiver 11 on the car side. When the driver leaves a car, a door is locked. If the insertion of the foreign object is detected by a sensor 6 when the alarm unit activating signal is transmitted from the transmitter 10, an alarm unit 4 such as a buzzer is driven by the CPU 9 to prevent a theft. The radio wave transmitted from a radio wave transmitter 110 is received by an abnormality warning means 111 added to the transmitter 10 to operate it.