PURPOSE: To make it possible to demonstrate the burglary preventing function with simple structure, without wrong operation and with long-term stability by providing a sensor for detecting the insertion of a key or the like into a cylinder lock and a remote controllable alarm driving means.

CONSTITUTION: A sensor 6 for detecting the insertion of a key or the like into a cylinder lock and an alarm driving means 12 operable by a portable remote control transmitter 10 are provided. If the sensor 6 and the alarm driving means 12 are put in the operable state at the time of an occupant and the like leaving a building, an alarm 14 is operated by the insertion of the key or the like into the lock. Moreover, this abnormal state is transmitted from a radio transmitter 50, so that burglar signals are sent out through an abnormality warning means 51 provided in addition to the remote control transmitter 10 as well as neighbors are informed of this state. On the other hand, if a door key is inserted into the lock in the released state of the sensor 6 and the alarm driving means, neither the alarm 14 nor the radio transmitter 50 is operated.