To make the creep rupture strength of a welding heat affected zone be hardly reduced, and to achieve long lifetime when the welded part structure of austenitic high-Ni steel products containing, by mass, 20-45% Cr is used for a long period of time at the temperature of 1,000C.

At least a welded part and an area in the vicinity thereof of steel products are subjected to an aging treatment before or after welding the austenitic high-Ni steel products for 0.5-100 hours at the temperature of 750-1,000C, and fine secondary carbide is deposited on the welded part of the steel products or a welding heat affected zone in the vicinity of a re-melted part of the steel products.