With extremely-thin-film and thin-film measurement, models are formed based upon a combination of film thickness, optical constants obtained using the dispersion formula, incident angle, etc., and the model and measured spectrums are fit by BLMC for a single layer of a structure with a certain number of iterations, obtaining information regarding the single layer. With thin-film multi-layer-structure measurement, models are formed for multiple layers of a thin-film multi-layer structure likewise, and fit by BLMC or EBLMC, obtaining information regarding the thin-film multi-layer structure. In either measurement, light is cast onto a thin film on a substrate to be measured while changing the wavelength as a parameter in order to obtain the spectrums .psi..sub.E(.lamda..sub.i) and .DELTA..sub.E(.lamda..sub.i) for each wavelength .lamda..sub.i, representing the change in polarization between the incident and reflected light. The measured spectrums are fit, obtaining the best model. The results are confirmed and stored, as necessary.