End portions of face plates 21 and 22 of a hollow extruded frame member 20 are connected with a rib 24 and these end portions are welded to end portions of face plates 11 and 12 of a hollow extruded frame member 10 using friction stir welding. A visible outline of a connection portion of the end portion of the face plate 21 and the rib 24 is constituted by circular arcs 31 and 32 which are recessed in the hollow extruded frame member 20. A visible outline of a connection portion of the end portion of the face plate 22 and the rib 24 is constituted by circular arcs 33 and 34 which are recessed in the hollow extruded frame member 20. The diameter of the circular arc 31 (33) is smaller than the diameter of the circular arc 32 (34). Accordingly, a structural body having a light weight structure can be obtained.