PURPOSE: To make the measurement highly accurate, by moving a projecting grating, and detecting the variation in the amount of light at a point to be measured as the phase of the Moire fringes by using a specified expression.

CONSTITUTION: The mesh shaped projecting grating 4 is moved in the surface direction at a constant speed, and light is irradiated from a light source 6 through a condenser lens 5. The transmitted light is projected on a body to be checked 1 through a projecting lens 2. The reflected light is focused on a secondary image pickup element 10 through a focusing lens 3 and a relay lens 9. Its output signal is supplied to an operating circuit 11. A triangular wave is taken out of each position of the mesh and approximated by a sine wave S. A phase standard sine wave S0 is generated through an input and output device 12. The difference between S and S0 is computed by using the specified expression. Said computation is executed for every address in the mesh, and the configuration of the object is determined. Therefore, the measurement of the configuration is performed highly accurately.