A first thermistor 8 and a second thermistor 9 are arranged forwardly and rearwardly of a thermopile sensor 5. A thermopile chip 55 is arranged and interposed between the first thermistor 8 and an integrated thermistor 57. A sensor cover is mounted in contact with front and side portions of a can portion 59 of a thermopile casing 56. A temperature or a radiant quantity of infrared rays on the front portion of the can portion is estimated from a temperature change of the integrated thermistor per second.