To enhance the sensitivity of an optical system to a minute flaw by providing a light source for emitting a light to a surface to be measured, and a branching means for branching the reflected optical path into a plurality of receiving side optical paths, and calculating the ratio or difference between the outputs from respective light receiving parts branched by the branching means.

Two light receiving parts 11, 15 are set by an opaque mirror 6 providing a linear output characteristic to the surface roughness of a glass surface 2, and the ratio of apertures of slits 10, 14 is changed, whereby the sensitivity to the flaw of the surface 2 can be changed. A light source 3 is of a repeated pulse-like emitting configuration, and a near infrared ray (a) is made into a parallel light (b) by a lens 5 to light a fixed area of the surface 2. The mirror 6 is set at an inclination ν of 45° to the reflected light P0 to branch it into two light receiving side optical paths P1, P2 by the mirror 6. Since the optical path P1 has the slit 10 having an opening 9 substantially the same size as the mirror reflected luminous flux, and the optical path P2 has the slit 14 having an opening 13 larger than it, the degree of the flaw can be shown by numerals by the output ratio between the both.