A spectroscopic ellipsometer is provided for measuring a small target surface with a high degree of precision. An irradiating optical system provides a polarized light to the surface of the target, while a detecting optical system is provided with a higher F-number for collecting the reflected light from the target surface to introduce it into the spectrometer for measuring a thickness of a thin film on the surface of the sample in accordance with the polarization state of change of the detected light rays.