Intrusion detection system

The intrusion detection system of the invention, in which three (2a,1a,1b) pyroelectric detectors are disposed in line with a interval and adjoining two (1a,1b) of the three pyroelectric detectors are electrically connected to cancel electrical charge generated by each pyroelectric detector, detects intrusion of an infrared ray radiating object such as a human body for example by output signals outputted from the adjoining two (1a,1b) and the other of the three pyroelectric detectors or by output signals outputted from pyroelectric detector (1b) disposed at the center and adjoining one (1a) and output signals outputted from the one (1b) disposed at the center and adjoining another one (2a) of these pyroelectric detectors, so that precise and secure intrusion detection is possible by reducing erroneous signals generated by those pyroelectric detectors due to variation of the atmospheric temperature and the like.