Recently, for the purpose of production rationalization, demand for CAD(computer-aided design) systems has been rapidly increasing. However, most of the CAD systems in mechanical design have mainly performed graphical processing, such as drawing. In this paper, we proposed an integrated computer-aided mechanical design system to support the design process as well as the drawing process. The system employs a mechanical-design-oriented description language called MODEL to design mechanical systems. To reduce the amount of descriptions in MODEL, we introduce knowledge bases for mechanical design. With these knowledge bases, the system can infer final designs from insufficient descriptions of objects at higher abstraction levels and complete them. Inference and knowledge representation schemes are discussed in detail. We also construct a prototype system and examine the effectiveness of our system.