PURPOSE: To correct easily the titled apparatus by opposing a black body part and a heat generating part of a calibrating body to a photodetecting part, obtaining correcting signals of background noise and a sensitivity error and calculating the signal obtained by opposing a transparent part of the calibrating body to the photodetecting part together with said corrected signal.

CONSTITUTION: The transparent part 121, the black body part 122 and the heat generating part 123 are made to oppose to a photodetecting part 111 of a two- dimensional infrared ray detector 11 at every revolution of the calibrating body 12 by 120°. In case of a correction of the background noises, firstly, the part 122 of the body 12 is opposed to the part 111, and a conversion to an electrical signal is performed at every image element of the detector 11. The signal is then stored in order in a storing part 15 from an A/D converter 13 through a switch 14. Next, the switch 14 is opened, and the part 121 is opposed to the part 111. The signal from the detector 11 is digitized and supplied in order into a subtractor 16. Here, the signal corresponding to the background noised stored in the part 15 is subtracted therefrom. As for the sensitivity error correction, the part 123 is opposed to the part 111, and the output signal is stored to a storing part 32 from the converter 13.