When an interpolation point is interpolated using a curve/curved surface, represented by control points, such as a B-Spline curve/curved surface or a subdivision surface, interpolation processing is performed in such a way that a moving vector for moving each control point is calculated using position information on interpolation points constituting the point group, as well as vector information on the unit direction vector that is set for each interpolation point, and the control point is moved in the movement direction and for the movement amount of this moving vector. By, repeating the generation step of a curve/curved surface that interpolates the interpolation point and the movement step of the control point, the position of a new control point is found that fits the curve/curved surface to the positions and the unit direction vectors of the point group.