A composite body and its manufacturing method, the composite body having first and second members (31,32), each having a structure comprising spaced first and second face webs (33,34) which provide opposite exterior faces of the member and at least one cross-web (35,36) connecting said face webs, said first and second members being shaped and adapted so as to be assembled together in a joining arrangement in which respective edges (34a) of said first face webs (34) are mutually adjacent for direct connection to each other while said second face webs (33) are mutually spaced apart by a gap, said edges (34a) of said first face webs (34) being joined by a weld bead (45) formed by friction stir welding, and said second face webs (33) being connected by a third member (60) welded to both said second face webs and bridging across said gap between them.