The design concept has been used confounding it with the commodity concept or the product concept because it has been not defined well yet. The purpose of this work is to try defining the design concept of Product design with referring to the definition by K. Sugiyama about Bridge design. In the first half of this paper, problems of the definition by him have been examined and considered from a view of Product design. It is found that viewpoints of time, added value, marketing and technology trend are necessary for the new definition. The extension of the definition was much considered by use of these viewpoints in the latter half. The accurate direction of the new definition of the design concept has been obtained from the consideration.