At a stage before manufacture (product design stage or manufacturing process design stage), influence of defect occurrence in a component or process on other components and processes is estimated, and defect importance is evaluated to support the manufacturing process design. Failure rates of components are previously stored in a database. At a product design stage, an assembly fraction defective is calculated for an assembly process of each component of the product. Component arrangement relations are extracted from product design information. A component fraction defective is calculated based on the assembly fraction defective and the component failure rate. A numerical value indicating an influence degree of each assembly process is calculated by multiplying component fraction defectives by coefficients based on of the component arrangement relations and the component fraction defective, for relating components in each assembly process. Thereby, influence degrees of each component and process are evaluated.