PURPOSE: To perform emissivity correction and linearization at the same time and to handle various kinds of radiation thermometers by calculating the ratio of the 1st and the 2nd input signals, multiplying the ratio by a coefficient, and calculating reverse logarithm.

CONSTITUTION: Radiant energy L from an object 1 of measurement is converted by a detector 2 into an electric signal and the 1st signal e1 of homochromatism or signals e1 and e2 regarding radiant energy of different wave are inputted to a logarithmic arithmetic means 3, which performs the logarithmic arithmetic of the ratio of both signals. Then the output of the arithmetic means 3 is multi plied by a coefficient (n) through a coefficient means 4 and a reverse logarithmic arithmetic means 5 performs reverse logarithmic arithmetic. The output of the arithmetic means 5 is processed through the polygonal line approximation of a polygonal line function generating means 6 to obtain a correct temperature signal. Further, the emissivity signal of an emissivity setting means 7 is inputted by a switch means S as the 2nd input signal of the arithmetic means 3. Conse quently, the emissivity correction and linearization are performed at the same time and this device is applied to both a homochromatic and a two-color ther mometer to use circuits in common.