1. A method of computing material requirements for a product by using a plurality of part computing units, comprising the steps of: judging a parent-child relationship among a plurality of constituent parts of the product; assigning said plurality of constituent parts to any of said plurality of computing units so that parts in the parent-child relationship are subjected to a part expansion computation by the same part computing unit, and executing a part expansion computation of said plurality of constituent parts, by using a plurality of constituent parts computing units without giving a low level code to each of said plurality of constituent parts of said product.