To provide a forming device of a square steel tube to form a skelp into an electric resistance welded tube of circular section, and then to re-shape it into a square steel tube having high accuracy of profile at a corner part.

This forming device comprises a measuring device 30, which is arranged on the outlet side of a square steel tube manufacturing line to automatically and correctly measure the profile of a corner part of a square steel tube 9, an arithmetic unit (a computer 1) for analyzing the measured value to operate the curvature, and an automatic position adjusting device (a process computer 5) to output the signal 8 for adjusting the relational position of a re-shaping roll 21, so that the curvature of every corner part agrees with the preset curvature, based on the operated result.