PURPOSE: To obtain a uniform bending angle over the entire length of a workpiece by calculating deflection quantity by fetching the detection output of a deformation sensor for an upper beam and a lower beam and controlling the driving of a plural actuator so that the deflection quantity goes to an optimum value.

CONSTITUTION: The detection outputs of all deformation sensors SU1 to SUn, SD1 to SDn are successively fetched by an NC device, and the deflection quantities of the upper beam 4 and the lower beam 5 are calculated from the detection output. Also, by comparing the deflection quantity of the upper beam 4 with that of the lower beam 5, whether or not the deflection quantity is optimum is decided. When the deflection quantity is not optimum, a command signal for a control valve is successively corrected by an NC device and each of hydraulic cylinders ACT1 to ACTn is successively corrected. Again, the detection outputs of the deformation sensors SU1 to SUn, SD1 to SDn are successively fetched the deflection quantity is calculated, and the comparison is made to decide whether the quantity is optimum or not. The operation is repeated until the deflection quantity is optimum.