To report the invasion of an illegal invader by touching a handle of a crescent-type at the time of lock and generating an alarm signal through a movable plate body to be separated at the time of unlocking.

The alarm system 5 is provided with a lock interlock switch, unlock switch and control part, etc., inside a body 6. When a resident closes the window sash and turns a crescent 4 in locking direction, a handle 4a of the crescent-type 4 is abutted on the top end part of a movable plate body 7 and pressed. Therefore, the movable plate body 7 turns on the lock interlock switch into alarm state. Next, when any invader illegally unlocks the window by inserting the arm from the outside into a hole pierced on glass, the top end part of the movable plate body 7 is released from pressure and the lock interlock switch is turned off by energizing force. When such a state is detected and the unlock button 9 is not depressed, the control part judges the illegal unlock, generates the alarm signal, rings a buzzer 8 and reports the existence of the invader to the resident, etc.