To reliably prevent the occurrence of surface cracking on a tube body upon bending.

When a tube body 1 is bent, there are arranged a top wall 21 having a constant width, which is allowed to face a tube body side face opposite to the bending direction and extends in the longitudinal direction of the tube body 1 while being separated from the side face by a constant distance, and a backing metal 2 having side walls 22 and 23. Both ends of the top wall 21 are provided with end walls 41 and 42 which are perpendicularly bent. The end walls 41 and 42 are joined to both ends of the tube body 1. The tube body 1 is bent in such a way that a neutral axis in bending extends in the longitudinal direction on the top of the cross section of the tube body side face facing the top wall 21 or at the vicinity thereof.