PURPOSE: To prevent the theft of an automobile by operating an alarm unit when the unlocking action of a door cylinder lock is detected by a sensor provided near the cylinder lock and operating the abnormality warning means of a receiver carried by a driver.

CONSTITUTION: A sensor 6 detecting that a door key 200 or another object is inserted to unlock a door cylinder lock 1 is provided near 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 mounted receiver 11. When the driver leaves a car, a door is locked. If the unlocking action of the cylinder lock is detected by the 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 56 is received by an abnormality warning means 51 added to the transmitter 10 to operate it.