PURPOSE: To eliminate a deviation in a focal position by a step of a sample or difference in reflectivity by consisting the titled mechanism of a slit which forms the light from a light source to an annular shape and inside and outside detecting parts which separate the light to an annular shape and including a detector which detects the reflected light from the sample into said mechanism.

CONSTITUTION: The light source 1 exists on the optical axis of an objective lens 4 and the light from the light source 1 is collimated to parallel rays of light by a condenser lens 8. Said beams are transmitted through the slit 2 having an annular transmission part. The light of the light source 1 is formed to the annular shape and is reflected 90° by a total reflecting prism 5 at this time. The reflected light is then passed through a matching lens 9, a (1/4)λ plate 6, a half mirror 7, and an objective lens 4 and is projected on the sample 16. The reflected light of the projected light traces the reverse optical path and arrives at the objective lens 4, the half mirror 7, the (1/4)λ plate 6, the matching lens 9, and the total reflecting prism 5. Since the reflected light is past the (1/4)λ plate 6 twice, said light passes the total reflecting prism 5 without being reflected 90° by said prism and is entered to the detector 3.