Conceptual design is the process to determine the basic concept, to define the overall function, to establish the functional structure, and to develop and complete the structure of the product, which is to be designed. In the existing conceptual design studies, however, it has not been verified that there is a theory regarding the product development process, with figuring out the correlation among Worth (W), Cost (C), and Satisfaction (S), and drawing the design equilibrium constant. Therefore, this paper presents the design equilibrium constant which explains the product development process, but can also be utilized in the conceptual design process. The design equilibrium constant is derived through defining the ideal product, its market competitiveness, and utilizing the process of the formulating Kano model. When utilizing the derived design equilibrium constant, it helps explain how the development process of the product makes progress in the market, and clarifies the change of an aspect of product life cycle.