PURPOSE: To measure an object contour according to actual arrangement by making a plane correction for contour data obtained from a fringe image via the deformation grating projection method.

CONSTITUTION: A beam through a grating 3 and a projection lens 4 after emission from a light source 1a, forms fringe images on the surface of a measurement object at equal pitches (i.e., deformation grating projection method). Then, the picture element data of the fringe images is inputted to a computer 11 through a CCD camera 7, a controller 9 and an input board 10. Thereafter, the grating 3 is moved by 1/4 pitch on the operation of a table controller 12 and the fringe images are similarly photographed. The data so obtained is repeatedly inputted to the computer 11 by four times and jointed across the entire surface of the object, thereby obtaining overall contour data. Then, this data is subjected to a plane correction, using an equation Z=a+bX+cY, where (a), (b) and (c) are conversion parameters, X and Y are the coordinates of contour data for analysis and Z is correction height.