To provide a manufacturing method for a square shape steel pipe in which the intermediate flat plate portion is made flat, and the size and shape of each right angle bent portion are made even.

Two portions near equivalent to groove portions in the width direction are bent into a right angle, and two portions inside the right angle bent portions 3 are bent into an obtuse angle to form an approximate square shaped steel pipe having an aperture between the equivalent to groove portions. While transferring the approximate square shaped steel pipe in the longitudinal direction, the group of forming rolls 42, 43, 44 of a roll forming means 40 is made to act on outer surface of four edges to make forming. At this time, corrective rolls 52 are made to contact with the inner face of four corners of the approximate square shaped steel pipe, the inner face of the four corners are corrected so as to have the same bending radius R. Welding is performed to a butting portion 11 formed by the right and left equivalent to grooves portions. The square shape steel pipe causes no corrugated or lower part bulging and curved state on the flat portion, capable of making the size and shape of the bending radius of the right angle bent portion even, excellent in quality, and in particular suitable in the case of large diameter and ultra large thickness.