PURPOSE: To evaluate a thin film from the envelope characteristic of the peak of an interference light spectrum obtained by composing reflected lights, and precisely determine the thin film information such as film thickness distribution with no destruction.

CONSTITUTION: The luminous flux from the light source 21 of a spectroscope 12 is halved by a beam splitter 22, one is reflected by a fixed mirror 24 and the other by a moving mirror 26. The reflected lights are composed by the splitter 22, the interference luminous flux is guided onto a material 14 to be measured by a reflecting mirror 16, and the reflected light is guided 20 to a detecting means 8. The means 18 outputs an interferogram signal and conducts Fourier transformation 30 to provide a spectrum signal. An envelop arithmetic means 32 determines the envelope of the interference spectrum waveform expressed on this spectrum. The wafer obtained by doping it by epitaxial growth or thermal diffusion is taken as the material 14 to be measured, and an evaluating means 34 conducts thin film evaluation from the envelop information. Thus, the thin film information such as film thickness distribution can be precisely determined with no destruction.