1. A three-dimensional model generation system for generating three-dimensional models, comprising: geometry-generating/editing means that is capable of changing an edge of a three-dimensional model into a fillet or a chamfer; three-dimensional data storage means for storing geometry data items associated with respective features generated or edited by said geometry-generating/editing means, on a feature-by-feature basis, and when any of said respective features is a fillet or a chamfer, further storing data concerning an edge erased as a result of generation of said any of said features, as data of a former edge associated with said fillet or said chamfer; feature management means for obtaining any of said geometry data items stored in a state associated with a corresponding one of said features from said three-dimensional data storage means, in response to a request sent from said geometry-generating/editing means, for obtaining three-dimensional data of said any of said features; and former edge data management means for obtaining said data of said former edge from said three-dimensional data storage means in response to a request sent from said geometry-generating/editing means to said feature management means, for obtaining three-dimensional data concerning said feature of said fillet or said chamfer, and generating three-dimensional display data from said data of said former edge.