Respective regional form information items obtained from regional interference fringe images corresponding to partial regions of a spherical surface to be inspected are transformed into regional synthesis form information items corresponding to a common coordinate system set for aperture synthesis by using a relationship among a polar coordinate system of the spherical surface, a plane coordinate system of an imaging plane, and the common coordinate system. Thus obtained regional synthesis form information items are subjected to aperture synthesis processing, so as to determine the overall form information of the spherical surface.