PURPOSE: To set a straightening roll to the optimum condition at the time of changing straightening conditions by making the last roll of a plurality of straightening rolls to bend a metallic bar alternately in the opposite direction take a working radius corresponding to the straightening condition and other rolls take prescribed working radii.

CONSTITUTION: A straightening device comprises a plied roll 1, a tension roll 5, an immovable straightening roll 6 and a movable straightening roll 7. The roll 6 has six rolls, a 1st roll from the inlet side has the same working radius as the tension roll 5 and the working radii of a 2nd roll to the 6th roll increase according to an arithmetical progression. The working radius of the movable sizing roll 7 is computed based on the bending moment of the metallic bar material 4 occurring while a plurality of straightening rolls pass to obtain the working radius which minimizes the warpage. Therefore, even though conditions of sizing are changed, the optimum condition can be easily set.