A small-sized measuring apparatus having a stray light suppressing capability for detecting the distribution of optical intensities on the cross section of a light beam having a predetermined wavelength contained in a light beam reflected from a measuring surface by entering a collimated light beam having a large cross sectional area into the measuring surface. A collimated light beam having a sufficient cross sectional area is entered into the interface between a dielectric block and a thin metal film formed on the dielectric block at an angle that satisfies the conditions of total reflection. A light beam having a predetermined wavelength is selected from the light beam totally reflected at the interface to detect the distribution of optical intensities for the selected light beam by the wavelength selecting section capable of eliminating stray light.