PURPOSE: To provide an intruder monitoring method capable of accurately recognizing the presence of an intruder.

CONSTITUTION: The presence of the intruder can be recognized when a signal from a spatial sensor operated by the motion of a person in a room is judged 1b and after that, an aperture sensor attached to window is judged 1d, and a signal from the aperture sensor is recognized after the lapse of delay time T. Also, when either the signal from the spatial sensor is present, or the signal from the aperture sensor is present, an alarm signal is transmitted from a central arithmetic unit to the room, and after that, it is judged that the intruder is present in the room when no response is received, or the degree of suspicion for intrusion is obtained based on detection information from the aperture sensor, then, the presence of the intruder can be recognized.