Abstract: A passive infrared ray sensor of the present invention includes a plurality of detecting elements 2 and 3 for detecting infrared rays of light emanating from an intruder H present in different detection areas A1 and A2 of a detection region A, and a detection processing unit 6 including a plurality of individual signal processing subunits 7A, 7B each operable to perform a signal processing subject to a detection signal fed from the respective detection element 2 or 3, whereby when the detection signal from at least one of the detecting elements 2 and 3 indicates a detection of a noise other than the intruder, the signal processing of such detecting element 2 or 3 is halted.