PURPOSE: To enhance the joining strength with a simple structure.

CONSTITUTION: A joining body 2 is formed of a square steel pipe of a prescribed length the outer diameter of which is set substantially equal to the bore diameter of square steel pipe columns 1a, 1b to connect, and a plurality of bolt holes 3 are provided on the upper and lower parts of each side wall of the joining body 2. On the other hand, bolt inserting through-holes 4 corresponding to the bolt holes 3 of the connecting body 2 are bored near each connecting end part of both the square steel pipe columns 1a, 1b, respectively. The connecting body 2 is fitted into the connecting end parts of both the square steel pipe columns 1a, 1b so as to straddle them, and bolts 5 are screwed to the bolt holes 3 through each through-hole 4 to fasten them, whereby both the square steel pipe columns 1a, 1b are integrally connected through the joining body 2. Further, the connecting end parts of both the square steel pipe columns 1a, 1b and the joining body 2 are fixed together by welding 8.