PURPOSE: To keep the same position relationship substantially all the time for a light receiving plate with respect to the surface of a plate-shaped body by changing the path of emitted laser light in correspondence with the different thickness of the plate-shaped body.

CONSTITUTION: The mirror-surface reflected light from an object under inspection obtained by the illumination with a laser light source 1 is received. A glass-plate light-path adjusting mechanism 4 is driven by the output of a light receiving device 8. Then, sufficient automatic compensating operation is performed for the deviation (m) of the thickness of the object under inspection and the rotation deviation at the outer surface of the object under inspection. In this way, the dislocation of the light path to the light receiving optical system for the emitted laser light due to the deviation of the thickness of the object under inspection, or the object under inspection having the different thickness or imperfect flatness of the object under inspection is eliminated. The height aligning of the thickness is not required, and the surface defect can be inspected quickly and highly accurately.