Roll forming a round tube to a square tube without applying excessive load to the raw tube scheduled portions to become corner portions and shoulder portions. This invention structures the curvature of the rotation axis direction of the forming roll surface in a way that in relation to other raw tube locations (straightening precedes for the raw tube locations adjacent to the corner portion scheduled locations of the square tube. It disposes stands of four-direction roll structure at the forming roll stand furthest upstream and furthest downstream sides and disposes stands of upper-lower and left-right two-direction roll structure between the upstream and downstream roll stands, and by adopting forming roll with a structure having a curvature for constricting the raw tube locations to become the shoulder portions adjacent to the corner portions of the square tube at a smaller curvature than that constricting the raw tube portions to become the side portion centers of the square tube cross section.