PURPOSE: To offset the effect of background heat radiation, by providing circuits, which generate a predetermined correcting signal voltage as the function of environment temperature based on the heat emissivity of a body whose temperature is measured, and subtracting the correcting signal voltage from the output signal of an infrared ray detecting element.

CONSTITUTION: The following circuits are newly added: an environment thermometer 6, which measures environment temperature from time to time; a correcting voltage generating circuit 7, which generates a correcting voltage αNo(To) {where No is the number of light quanta, which are reflected by the surface of a body whose temperature is measured and inputted to an infrared ray detecting element 1, in a background radiation} corresponding to the background heat radiation, which is obtained by inputting and computing the measured value of the thermometer 6, the background emissivity 2, and the heat emissivity 1 of the surface of the body whose temperature is measured; and a correcting circuit 8, which subtracts the voltage of the correcting voltage generating circuit 7 from the output voltage of the infrared ray detecting element. The output voltage of the correcting circuit 8 corresponds to the true temperature of the body to be measured. As a result, the effect of the background heat radiation is offset, and the temperature of the body to be measured can be accurately measured without contact.