To improve the precision of temperature compensation in an infrared sensor and obtain a sharp image, a correction is applied to a variation in output voltage (referred to as “background infrared radiation absorption intensity distribution” below) due to intensity distribution of background infrared radiation, which is light other than the incident infrared radiation on the infrared sensor, and the temperature characteristic of each individual bolometer constituting the infrared sensor. That is, the temperature of the infrared sensor is measured as a first temperature, a correction value for the output voltage of each bolometer is found by referring to a table, which indicates the background infrared radiation absorption intensity distribution versus the temperature of the infrared sensor, as well as the first temperature, and the variation in output voltage is corrected.