Welded high-strength steel structure with excellent SSC and corrosion resistance in a wet carbon dioxide environment having a low H.sub.2 S concentration. A structure having a Vickers hardness of at most 350 and a joint tensile strength of at least 620 MPa and composed of a base metal comprising a martensitic stainless steel with a martensitic or tempered martensitic structure containing 0.001 to 0.05% C and 9 to 14% Cr and a weld metal comprising an austenitic-ferritic stainless steel having a duplex structure composed of austenite and 35 to 75% by volume of ferrite and containing 22 to 27% Cr,7 to 11% Ni and 1.5 to 5% Mo, and at most 0.03% C as an impurity.