SQUARE TUBULAR STRUCTURE

PROBLEM TO BE SOLVED: To provide a square tubular structure in which a shape of a backing strip is simplified, and a space between a corner part of a square tubular work-piece and the backing strip can be reduced.SOLUTION: A backing strip 30, which is provided to a junction between a square steep tube (work-piece) 10 and a pedestal 20, is formed in a shape of plate having a length substantially equal to a length from one corner part 11 to the other corner part 11 which are connected to both ends of a flat plate part 12. In the backing strip 30, both ends thereof respective face said one corner part 11 and the other corner part 11, and a central part between the end facing the one corner part and the end facing the other corner part contacts the flat plate part. In the backing strips 30 neighbouring to each other, the ends most approach each other at the corner part 11. Among the neighboring backing strips, the end of one backing strip 30 enters to a deeper position in a bending portion of the corner part 11 than the end of the other backing strip 30, and the end of the other backing strip 30 contacts the one backing plate 30 or faces the one backing plate 30 with a predetermined space therebetween.