To achieve a simply configured and accurate fire detector, which solves the problem wherein power consumption is increased, when a large-scale circuit such as a multiplier or a high sensitivity phase detector is incorporated, and nearly at the zero level of a detection signal, noise or time delay between each of detection signals has a considerable effect, that can result in a mismatch between phases of light, even if the light is from the same light source.

Only when the detection signal from a photodetection means reaches a predetermined level, the sign components of each of detection signals detected by the plurality of photodetection means are compared by a sign comparator, composed of an arithmetic logic device for exclusive OR and an integrator.