Radiated light with a specified wavelength from a material (21, P, 201) is detected and a first parameter corresponding to the emissivity ratio is obtained from the detection signal. Since the emissivity takes on different values according to the condition of the surface of the material, the first parameter changes depending on the surface condition of the material. There is a correlation between a physical value indicating a condition of the material surface and the first parameter. The correlation remains equivalent even if a second parameter corresponding to the physical value is used instead of the physical value itself (for example, an optical physical value such as reflectivity and absorptivity, the thickness of a film formed on the material surface, the surface roughness, and the degree of galvannealing). As an example of the parameter corresponding to the physical value, there is the logarithmic ratio between emissivities (ln ε a /ln ε b ) corresponding to the temperature in the vicinity of the surface. Therefore, a second parameter can be obtained on the basis of the correlation and a physical value can be obtained. When the emissivity or logarithmic emissivity ratio is used as the second parameter, the temperature in the vicinity of the material surface can be obtained from the second parameter and the detection signal.