PURPOSE: To realize a simple optical measurement for a minute deformation, by deforming the surface of a sample to be measured into a concave or convex surface and then detecting the interference fringes which are caused by irradiating the coherent light to the deformed surface of the sample.

CONSTITUTION: A sample 1 has a mirror surface having the reduced diffused reflection with a deformed concave face. The coherent light beams 2 and 3 are projected to the surface of the sample 1, and a screen is set at the point H where the reflected beams cross to each other. Thus the interference fringes are caused to show the concave and convex contour lines. So is with the convex deformation caused on the surface of the sample 1. As a result, a minute deformation can be measured in a simple and optical way.