PURPOSE: To easily and quickly and prepare a bending data with a high precision when a work is bent by a bending machine at a specified bending position to make a box body.

CONSTITUTION: Data relating to the cross section shapes of the front direction and side direction of the box body as a product are inputted and data relating to the opposite shape at a corner part of the work are selected from a pattern prepared in advance, data relating to the developed size of the box body in consideration of the elongation allowance of the work 1 based on these data are operated and further data relating to positioning positions and bending positions by positioning blocks 4a-4e of the work are operated. Furthermore, data relating to the positions of the corner parts are made so that they can be corrected based on experience values.