PURPOSE: To simplify the computation and means for a straightening quantity by bending a lengthy linear metallic member axially in advance to a bending angle small or large to a reference angle of bending and making straightening deformation only from either the small angle or the large one.

CONSTITUTION: To take an example, a door sash 1 is manufactured by bending axial parts S1-2, S1-3 extending to the right and left centering an axial part S1-1 nearly in the middle of metallic member S1, with a bending machine. In that case, before each axial part S1-2, S1-3 are bent, they are arranged to either bending angles β1, β2... in a small range beyond a reference angle α to a specified bending reference angle line 0 or bending angles in a large range within the reference angle α. According to this axial bending, the metal S1 bent excessively axially to the minus side scatters on the minus side of the straightening target value 0 and axial bending is carried out from a state bent excessively axially to the minus side toward the straightening target value 0.