PURPOSE: To measure the nature of a surface easily without contacting on the basis of variation in the quantity of light when the position relation among a light source, a photodetector, and the surface to be measured, by irradiating the surface to be measured with light and then detecting its reflected light.

CONSTITUTION: Light from a light source 21 is passed through a lens 22,a slit 23, and a lens 24, reflected by the surface of a body to be measured, and photodetected by a photodetector 30 through a lens 27, a slit 28, and a lens 29. In this case, the angles θ1 and θ2 between the incident light beam and reflected light beam, and the normal of the surface to be measured are equal to each other. The optical system or surface to be measured is rotated around the normal axis at ≥90° and variation in the quantity of reflected light with the angle of rotation is measured to find the direction of a work streak, etc.