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

CONSTITUTION: A sensor 6 for detecting the insertion of a key or the like into a cylinder lock is provided in the vicinity of the cylinder lock as well as an alarm driving means 12 operable by a portable remote control device is 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 through a control means 9 by the insertion of the key or the like into the lock. If the operation of the sensor 6 and/or the alarm driving means 12 is released previously by the occupant and the like, the alarm 14 is not operated even by the insertion of the key or the like into the lock.