To detect abnormality even when plate glass is simply broken without moving the plate glass toward a fixed side member by forming a reflector for reflecting laser light L on the end face of the plate glass, and providing a light emitter and a light receiver for the laser light inside the plate glass.

On each end face of plate glass 3, a reflector 5 is formed for reflecting the laser light L. Inside the plate glass 3, along the glass surface of the plate glass 3, a light emitter 6 is provided for radiating the laser light L and a light receiver 9 is provided for receiving this laser light L. Thus, when the plate glass 3 is broken, the light emitting plane of a broken part is usually formed in irregular direction. Therefore, the optical axis of the laser light L is disordered by irregular reflection, the laser light L is instantaneously imbalanced in the middle inside the plate glass 3 or interrupted, and the stable quantity of light does not reach the light receiver 9. Therefore, the output voltage of the light receiver 9 fluctuates, abnormality is detected, and an alarm 12 is operated by a controller 11 to which a detecting signal is inputted.