To easily reinforce a pipe by diagonally inserting an axial member provided with a taper so that the plate thickness at the longitudinal central part becomes thinner than the plate thickness at the ends, in a square reinforcing steel pipe.

A reinforcing pipe 1 is a square steel pipe having a square section, and an axial member 2 is diagonally inserted. Symmetricly tapered faces 3a, 3b, 3c, 3d are formed at both faces in the axial members 2 so that the plate thickness of the longitudinal central part is thinner than the plate thickness of the ends. Only the section of the ends of the reinforcing steel pipe 1 is deformed to have a diamond-shaped section so that the clearance at the center becomes equal to the clearance at the ends. The clearance is the perpendicular length vertically extended from the corner end in the sectional area of the axial member 2 to the inside face of the reinforcing pipe 1. It can be zero. In this way, the ends of the axial member can be easily reinforced and the buckling stress against the axial member 2 can be made nealy even at longitudinal arbitrary sections and hence, the bracing member showing an ideal compression collapse mode can be obtained.