PURPOSE: To set an accurate set-up by predicting the wear amount of a roll at the external contact point between the roll and a tube to be straightened and setting a roll position and an oblique angle of the roll based on this predicted value.

CONSTITUTION: In a method for straightening the tube by a rotary straightner, a wear amount at the external contact point between the roll 2 and the tube 1 to be straightened is estimated based on the wear amount in the central part of the roll 2. Then, a quantity of offset, a quantity of crush and the oblique angle θ of the roll are preset based on the predicted value. In this way, since the roll position and the oblique angle θ of the roll are adjusted finely after they are preset, the straightening effect to the bend and ellipticity of the tube is excellent and roll mark flaws can be prevented from being generated.