To provide an automatic straightening method capable of straightening a bend of an electric resistance welded tube in the line based on the measurement data by automatically measuring the bending direction and bending degree of the electric resistance welded tube accurately in the line before the regular size cutting after the high-frequency welding.

In the tube manufacturing line of the electric resistance welded tube after butt-welding by high-frequency the steel strip end portion continuously shaped to a cylindrical form through forming rolls and followed by the straightening and cutting process, three two-dimensional displacement gages 21 are located in the exit side of Turk's head stand 22 with equal space and each two-dimensional displacement is simultaneously measured at each position and, based on the three measurements, the bending amount of the electric resistance welded tube is calculated. Using the bending amount, driving of motors 29, 30 which transfer the horizontal roll 23 and the vertical roll 24 of Turk's head stand 22 is feed-back controlled and the bending amount of the electric resistance welded tube is automatically straightened by adjusting the position of Turk's head roll.