An object is to reduce film thickness measurement error. Illumination light having different wavelengths is radiated onto a plurality of samples in which thin films having different film qualities and film thicknesses are provided on substrates, evaluation values related to the amounts of transmitted light when the illumination light of each wavelength is radiated are measured, film thickness characteristics, showing the relationship between the evaluation values and the film thicknesses for each film quality, are formed at each wavelength based on the measurement results, and among the film thickness characteristics, a wavelength at which a measurement difference between the evaluation values caused by the film qualities is in a predetermined range is selected.