<p>Light of an observation wavelength range is irradiated upon a sample object to measure spectral reflection ratios, and an waveform is developed from the spectral reflection ratios. Based on the total number of peaks and valleys found in the interference waveform and two wavelengths specified within the observation wavelength range, possible ranges for the film thicknesses of the respective transparent films are determined. While changing tentative film thicknesses of the respective transparent films each by a predetermined film thickness pitch within the film thickness ranges, a deviation between theoretical spectral reflectance and measured spectral reflectance with respect to the tentative film thicknesses is calculated to thereby find a film thickness combination which causes the deviation to be minimum. <IMAGE></p>
