PURPOSE: To prevent unlawful unlocking by detecting the opening of a shutter of an insertion port of a cylinder lock by a shutter opening detection means, and starting a timer to break alarm signals by an alarming means when it continues for specific times.

CONSTITUTION: A shutter 73 to prevent the incursion of rainwater, etc., is provided to a key insertion port 72 of a cylinder lock 7, and a shutter sensor 5 is provided between a guide 74 and the shutter 73. Detection signals of the shutter sensor 5 are inputted to monitor the state of the shutter 73, and when the opening of the shutter 73 is detected, a timer of a control circuit 2 starts. After that, actuating signals are outputted to an alarm unit driving circuit 3 and a message output circuit 4 from the control circuit 2, and an alarm unit 3a and a speaker 4a are actuated to break alarm signals. According to the constitution, burglaries caused by unlawful unlocking can be prevented.