PURPOSE: To prevent deterioration of detection sensitivity that could be caused by variation in the amount of light incident on a surface to be inspected and/or decrease in the gain of a light receiving system by providing between a light source and an illuminating optical system a monitor portion for detecting the amount of shift and angular deviation and a control portion for correcting them.

CONSTITUTION: A laser beam from a light source 1 is split by the beam splitter 9a of a monitor portion M1 and part of the beam reaches a beam position detecting element 10a and the portion of the beam transmitted through the splitter 9a is further split by a beam splitter 9b and reaches a beam position detecting element 10b and the position of each split beam relative to the optical axis is detected. An arithmetic circuit 11 computes the amount Δ of shift and angular deviation α relative to the reference optical axis of the laser beam according to the outputs of the elements 10a, 10b. A control portion C corrects the amount Δ of shift by operating a motor- driven halving 12 using a drive portion 13 and corrects the angular deviation δ by controlling a polarizing prism 14 using a drive portion 15, so the shift and the angular deviation are eliminated before the beam impinges on the illuminating optical system, and the amount of illuminating light applied to a surface 6 to be inspected is not reduced and the detection sensitivity is maintained.