In the initial process of exterior design of a product, designers often get the idea of a designing product form as silhouette lines and boundary lines. Therefore, if the 3D model is constructed from silhouette lines and boundary lines, designers will be significantly assisted in the process of external form design. This paper describes a system to construct a 3D model with complex curved surfaces from silhouette and boundary lines. The silhouette and boundary lines can be easily given by a computer mouse or a tablet and are then approximated by cubic Bezier curves linked with curvature continuity at the connected points. A Bezier surface is defined from the four lines as the silhouette or boundary by an originally developed algorithm. The 3D model is then constructed by Bezier surfaces with curvature continuity on the connecting lines. Since the system does not obstruct designer thinking due to simple operations of the system, designers can elaborate and confirm the idea by considering and examining the form indicated by the system.