We present a framework for building a knowledge-based system that can consistently and automatically utilize graphs, tables and formulae as a knowledge source in mechanical CAD. In this paper, we describe both design object and design process using constraint networks which consist of constraint assemblies, constraint objects and constraint modules. The design object is described by a hierarchical structure of the constraint networks, and the design process is represented by constraint propagation between them. We introduce the concept of functional constraint in the constraint network and show that the database search for design variables can be carried out automatically. The usefulness of this system is illustrated by the application to the design of a gear system.