50 kgf/mm²-class high-tensile steel plate produced by newly developed TMCP (thermo-mechanical control process) is superior to conventional rolled steel plate in weldability (resistance to low-temperature cracking) as well as in toughness of HAZ of the high heat-input welded joint. However, HAZ of high heat-input welded joint softens because of the low carbon equivalent. The authors investigated the tensile strength of the welded joint with soft HAZ in ship hull and other heavy steel structures. As a result, it was found that even with the maximum heat-input welding process prevalently in use in most of the domestic ship-yards, the tensile strength of the welded joint thereby produced in wide plate would be 50 kgf/mm² or more and approximate that of the base metal when Ceq. _≥0.23%.