PROBLEM TO BE SOLVED: To automatically measure the irregularity and vertical displacement of fabricating surface of a product during fabrication which is not a mirror surface but a diffusing surface of an object to be measured by non-contact, with high precision and in real time.
SOLUTION: The interference pattern on concentric circles caused by reflection light from the measuring surface, that is, the diffusing surface and a reference surface is read with an optical device. The moving quantity of sinking in circle center direction or floating from the circle center is calculated and the displacement is given to the reference surface in the direction vertical to the surface so as to cancel the moving quantity. As the displaced quantity is the same as the displacement of the measuring surface in the direction vertical to the surface, the displaced quantity is shown as the displacement of the measuring surface in the direction vertical to the surface. In this manner, automatic measurement becomes possible.