Sharing and understanding the product design, design information and design rationale integrated CAD system is needed for cooperative design. This paper proposed a method to support finding influential relations between parts of design object. First, data structure of influential relation was defined. This data structure is composed of quantity size and unit symbol of attribute/property value, and their relation. Then, a method to extract influential relation from product design case database was proposed. Finally, the case-based design support system for derivation of influential relations was developed, and case studies are done by using the system. The result of the case study showed that the proposed system can help designers to find and set the influential relations between parts.