To enable sections to be joined with each other from one surfaces by joining the ends of first hollow sections projecting triangularly to the opposite sides with each other and disposing second hollow section having the triangular part located in the joining part of the first hollow section to join the ends of the second hollow sections with each other.

A plurality of inner face side aluminum hollow sections 10 are arranged width wise of the extruding direction of the section, i.e., in the direction perpendicular to the longitudinal direction of a vehicle (circumferential direction of the vehicle) to have the ends joined with each other by welding. The outer surface side aluminum section 20 projects triangularly to the outer surface side between the ends in the width direction and has the flat top. The aluminum hollow sections over lapping the aluminum hollow section 10 are welded to each other while being also welded to the aluminum hollow section 10.