PURPOSE: To surety detect trespass and to prevent the occurrence of an erroneous alarm by providing two different detectors in the same protected area and giving an alarm only at the time when both detectors are operated in a specific mode.

CONSTITUTION: If a glass window is pushed, broken, or holed in a protected area, a crash sound of glass is emitted, and the pressure in the room is varied. This sound is recorded by a glass break detector 30, and the pressure variance is recorded by a pressure detector 10. Since the propagation speed of the pressure wave is lower than that of the acoustic wave, the pressure variance is detected with a delay of about 0.1sec. For the purpose of preventing an erroneous alarm, the glass break detector 30 is constituted as a microphone, and sounds on the outside of the frequency range of the sound caused by glass break are suppressed through a band pass filter 35. A band pass filter 15 is used to transmit only the variance peculiar to glass break. Further, an alar, is given in response to the variance peculiar to glass break by the delay time of a time delay circuit 45.