A product design and process design device includes: a storage unit that stores design specification information containing the tolerance of a dimensional variation at any site of an assembly product, part dimension error information, assembly facility error information, assembly tolerance information, the part dimension error information, and the assembly facility error information, part manufacturing cost information, and facility investment cost information with respect to the assembly facility error information; a tolerance distribution processing unit that uses the assembly tolerance information to generate proposed tolerance distributions that differ in distributional combination with respect to each error, and calculates part manufacturing costs and facility investment costs; a facility planning processing unit that determines a proposed tolerance distribution, with, as an evaluation index, total production costs respectively including the sums of the part manufacturing costs and the facility investment costs calculated; and an output unit that outputs the proposed tolerance distribution.