A shape data specifies the shape of an elongated product within an overall coordinate system. Local coordinate systems are defined on respective cross-sections of the elongated product in the overall coordinate system. The local coordinate system is designed to represent a stable die of a bending apparatus. The images of the elongated product protruding forward from the stable die within the respective local coordinate systems serve to specify the position of a movable die of the bending apparatus. The specified position reflects the deformation induced in the elongated product between the stable and movable dies. An ideal movement amount of the movable die can be determined based on the determined positions within the local coordinate systems. The determined movement amount can be utilized to prepare the control data for the bending apparatus. When the prepared control data is supplied to the bending apparatus, an ideal movement of the movable die can be achieved so as to effect bending on the elongated workpiece at a higher accuracy in accordance with the shape data.