To obtain information adapted to an actual work by digitizing the shape of two or more curved surfaces to be compared, calculating the characteristic values of the shape about the numerical values of each curved surface or the difference between the numerical values of each curved surface and executing the comparison and judgement of the shape of the curved surface using the obtained characteristic values of the shape.

The positions of representative points on the curved surface are beforehand decided as the numerical values by which the shape of the curved surface is expressed and the distance (height) from the reference surface at those points is used. As the representative points on the curved surface, a total of nine points of four vertexes of a rectangle, midpoints of four sides and center are adopted. Even these nine points are enough to digitize the shape of the curved surface in the case of a simple shape. As the representative shapes (deformation mode), five deformation modes are used. The mode 1 respectively expresses bending deformation in the X-axis direction, mode 2 bending deformation in the Y-axis direction, mode 3 bending deformation (twist) in the diagona direction, mode 4 inclination in the X-axis direction and mode 5 inclination in the Y-axis direction. The shape used practically can be approximately expressed by these characteristic values.