PURPOSE: To make it possible to prevent the unlocking of a door with a false door key, etc. by installing a sensor which detects the insertion of a door key, etc. in a door cylinder lock and actuating an alarm with that output signal and the output signal from an alarm driving means.

CONSTITUTION: In a door cylinder lock 100 consisting of a cylindrical outer cylinder 1 and an inner cylinder 2 with a key insertion slit 3, which is pivotally mounted therein, a sensor 6 comprising a photoelectric element, etc. with detects the insertion of a door key 200 or other stuff in the inner cylinder 2 is installed on a hole 4, 4 of the outer cylinder 1. An alarm driving means 12 so constructed to emit a signal for driving an automobile mounted alarm 14 is installed. The means, for an example, consist of a remote controlled transmitter 10 and a receiver 11. When an output signal is emitted from the sensor 6 while an output signal is being emitted form the driving means 12, the alarm 14 is operated by a controlling means 9 to prevent the theft.