PURPOSE: To efficiently make highly precise straightening by deciding the optimum position for straightening and a straightening amount at each position for straightening.

CONSTITUTION: A deviation from a target line, curvature, etc., at each measuring point are calculated; these values are each taken as a factor for the fuzzy theory; the position for straightening is decided by the fuzzy theory so that the bend of a workpiece W is within a tolerance; and the straightening amount at each position for straightening is simultaneously decided so that the absolute value of the maximum deviation against the target line is minimum. Then, while a simulation is performed, a position for the required irreducible minimum straightening and the straightening amount at each position for straightening are decided in advance, and the straightening is performed.