End portions of face plates 111 and 112 of a frame member 110 are connected by a rib 116A. To protrusion chips 117b1 and 117b2 end portions of face plates 101 and 102 of another frame member 100 are overlapped. The corners between the rib 116A and the rear faces of the face plates 111 and 112 are in the form of a convex circular arc 131 which protrudes into a hollow portion 119 of the frame member 110. Both end portions of the circular arc 131b1 are connected to the rib 116A and the face plates 111 and 112 by two smooth recessed circular arcs 132b1 and 133b1. Accordingly, since a flow-out of metal does not occur from this portion, the cross-section can be small and a light weight structure can be obtained. A structural body formed of hollow frame members which are joined by friction stir welding provides a light weight structure.