A light beam irradiating optical system is associated with a dielectric material member having a surface, on which a thin film layer has been formed, a sample being brought into contact with a surface of the thin film layer. The light beam irradiating optical system produces and irradiates a light beam to an interface between the dielectric material member and the thin film layer. The light beam is constituted of light beam components, which have various different incidence angles with respect to the interface, and which have intensities varying in accordance with the incidence angles. A single measuring detector for outputting a signal representing an intensity of an entire area of received light is secured and located so as to receive the light beam having been reflected from the interface.