A method for deriving the position of a pushing roll, applied even when there is a difference between the actual processed shape and a theoretical solution (a numerical analysis solution) due to changes in a state of a processing machine or the bending characteristic of the material to be processed. The rolls have a pyramid-like shape, and the operation amount of a pushing roll is changed while continuously feeding a material, thereby bending the material. Also, for each position of the fixed pushing roll, the radius of curvature of the material is measured and the bending characteristic is grasped in advance. From the design shape, the radius of curvature and the operation amount for bringing the pushing roll into contact are obtained. The operation amount of the pushing roll is then determined.