In the design process of the external form of a product, sketching is applied for creating and embodying idea. Sketches are able to reflect the aesthetic sense of designers the most easily, making them the most efficient design tool. Sketches can be classified into thumbnail sketches, rough sketches, and rendering sketches according to the embodied idea. Rough sketches are used for developing the idea. In this paper, a system to support the rough sketching process and evaluate the designed product shape using 3D models constructed from the rough sketch is described. This system has a function for extracting and generating sketch lines drawn by a designer, a function for interactively modifying sketch lines, and a function for constructing 3D models from rough sketches. In order to support rough sketching, algorithms which automatically extract/generate the desired sketch-lines are proposed and implemented as a system. A method to construct 3D models form sketch lines is also proposed and implemented as a system.