<p>An infrared ray detector is disclosed in which four infrared ray detecting elements (8a-8d) are arranged corresponding to window materials (4) transmissible to infrared rays in a sealed case (1), of which openings (3) are closed with said window materials (4). Respective optical filters (9a-9d) transmissible to infrared rays having respective wavelength bands absorbed by a HC gas, a CO2 gas, a CO gas and a standard gas are arranged between the respective window materials (4) and said respective infrared ray detecting elements (8a-8d). The central wavelength and a half-bandwidth of said wavelength bands of infrared rays transmissible through the respective optical filters (9a-9d) are set within a range of the following standard values +/- about 5 %: Optical filter for use in HC: Central wavelength: 3.4 mu m; Half-bandwidth: 8.6 % Optical filter for use in CO2: Central wavelength: 4.3 mu m: Half-bandwidth: 4.2 % Optical filter for use in CO: Central wavelength: 4.7 mu m; Half-bandwidth: 8.6 % Optical filter for use in the standard gas: Central wavelength: 3.8 mu m; Half-bandwidth: 3.0 %</p>
