Abstract: The improved superalloy that possesses all the characteristics required of the high-temperature structural material of high-temperature gas-cooled reactors (i.e., high-temperature strength, corrosion resistance, good productibility, good hot workability and resistance to embrittlement due to thermal aging) consists essentially of 16-28% Cr. 15-24% W (provided that Cr+W=39-44%), 0.01-0.1% Zr, 0.001-0.015% Y, 0.0005-0.01% B, up to 0.05% C, up to 0.1% Si, up to 0.1% Mn (provided that Si+Mn.gtoreq.0.1%), up to 0.1% Ti, up to 0.1% Al and up to 0.1% Nb (provided that Ti+Al.gtoreq.0.1% and Ti+Al+Nb.gtoreq.0.15%), with the balance being Ni and inevitable impurities and all percentages being on a weight basis.