PURPOSE: To threaten an illegal trespasser and indicates an abnormal state by sounding an alarm bell by using a backup power source incorporated in the bell device when an open circuit is detected.

CONSTITUTION: When it is judged from the analytic result of an abnormality signal detected by a detection sensor group A that alarming needs to be commanded, a control circuit B2 sends a bell sounding command to the bell device C through a sounding control line E2 and the bell device C having received this bell sounding command sounds the alarm bell C1 through a bell sounding control circuit C2. At this time, the electric power for sounding the alarm bell C1 is supplied from a main power circuit B3. If the illegal trespasser disconnects the bell connection line E between a guard monitor device B and the bell device C, on the other hand, an open circuit detecting device C3 incorporated in the bell connection line E operates to sound the alarm C1. At this time, the backup power source C4 in the bell device is basically used as the power source for sounding the alarm bell.