To hold parallelism of a filter and a pyroelectric element, to expand an angle of visibility, and to improve detecting accuracy by forming both element base stands as a similar structure having air vents for communicating the inside/outside the annular part, and attaching the pyroelectric element to the filter through the second element base stand.

An element base stand is a four-corner cutoff square ring shape, and is composed of first/second element base stands 51, 52 of a similar structure. Air vents 57, 58 communicate the inside/outside of the annular part. A pyroelectric element 4 is mounted on the surface side of a circuit board 6 through the element base stand 51 so that the pyroelectric element 4 is attached to a filter 3 through the element base stand 52. A pyroelectric intrared ray sensor is arranged by turning a monitoring window in the direction for monitoring a fire, and when a fire is caused in a monitoring area in an angle of visibility, infrared rays from a flame are made incident through the monitoring window, and are detected by the pyroelectric element 4 to be converted into an electric signal so as to be compared with a reference value by a comparing circuit through an amplifying circuit of the circuit board 6 to judge the occurrence of the fire. Measures of an announcement of the fire occurrence and the prevention of the fire are taken on the basis of this judging result.