1. A method of approximating shape data to a desired fineness degree, comprising the steps of: designating information concerning a degree of fineness of approximation for a desired portion of the shape data, wherein importance information is used as the information concerning the degree of fineness of approximation; and hierarchically approximating the desired portion of the shape data using the designated information concerning the degree of fineness of approximation, wherein evaluation values for one of surfaces, edges and apex points of each portion of the shape data representing a degree of importance in representing the shape data, are found, and wherein the step of approximating is controlled using said evaluation values and the importance information based on designation by a user.