To objectively measure an acute angle bending or an obtuse angle bending not through workers' hands by detecting an unilateral elongation value, an outer radius of a workpiece during bending.

During bending a workpiece, from a plurality of workpiece profile coordinate values (step S4) detected by a workpiece profile coordinate value detecting means and a workpiece plate thickness input in advance, a plurality of workpiece neutral axis coordinates are computed (step S10). By the sum of the distance between coordinates of plural workpiece neutral axes, the workpiece neutral axial length is computed (step S11). From a tangential intersecting point of both flanges of the workpiece by the workpiece profile coordinate values, a flange length is calculated (step S12). Based on the calculated flange length and the computed workpiece neutral axial length, the bending is executed while computing the elongation value of the workpiece (step S15).