PURPOSE: To easily generate a development even in the case of other product than a rectangular parallelopiped.

CONSTITUTION: From plural kinds of reference shape patterns registered in advance as a shape of one principal surface of a product, a desired reference shape pattern is selected (step S1). Also, from plural kinds of bending shape patterns registered in advance as a cross sectional shape of the part to which bending is performed, a desired bending shape pattern is selected (step S3). To the selected reference shape pattern and bending shape pattern, dimensions of each part are given by finish dimensions (steps S2, S4). A material of a sheet metal to which bending is performed and plate thickness are inputted (step S5). With respect to a first development (step S6) in which the reference shape pattern and the bending shape pattern are coupled, a correction is performed so that bent pieces are not overlapped and a second development is obtained (step S7). Based on the material of the sheet metal and its plate thickness, elongation dimensions of the bending part are derived (step S8), and the dimensions of a second development are corrected (step 9).