To provide a light and highly rigid structure.

A hollow structural member is formed by joining two face plates 21 and 22 with ribs 23A and 23B in the form of a trussed structure. The end of the facing plate 21 is located near the summit of the truss of the end of the hollow structural member. The end of the other face plate 22b is provided to protrude from the end of the face plate 21. The face plate 22b is welded from above with a friction stirring joining method. Then a connecting member 30 is placed on the face plate 21 and joined from above with a friction stirring joining method. A load is received by the ribs 23A and 23B during the friction stirring joining. The whole structure becomes a trussed structure resulting in a light weight structure with a high rigidity.