PURPOSE: To facilitate a data file maintenance job by providing an item check table where the correspondence is secured among the product codes, the design data codes, and the estimation data codes and then carrying on repetitively the design and estimation of the products.

CONSTITUTION: An item check table 3 uses the internal parts codes as keys in order to refer to the parts shape codes and parts No codes. These shape codes and No codes contain plural codes in accordance with the shapes, the materials, the colors, the prices, etc., of parts. A CAD program 2 refers to the table 3 to obtain the parts shape codes and the parts No codes. Then a parts graphic data file 4 is retrieved by the parts shape codes for extraction of the design data. Furthermore a parts data file 5 is retrieved by the parts No codes for extraction of the estimation data. Thus the file control is facilitat ed for updating of the data.