To provide a method and a device for forming a square column wherein the configuration accuracy of the formed square column can be higher than that by a conventional method and device.

In forming an electric resistance welded tube of a circular cross section into a square cross section by a multistage re-shaping roll unit, on-line measurement is continuously done on the circular cross sectional shape of the electric resistance welded tube, the cross sectional shape in process of forming into the square column and the square cross sectional shape and corner shape just after the completion of re-shaping. The setting conditions of the re-shaping rolls at each stage are determined according to the overall information on the measured values, and the forming process is performed under those conditions.