A Ni-base superalloy consisting essentially of, by weight: 0.05 to 0.20% C, 20 to 25% Co, 15 to 25% Cr, 1.0 to 3.0% Al, 1.0 to 3.0% Ti, 1.0 to 3.0% Nb, 5 to 10% W, and at least 42.5% Ni, the combination of the [Al+Ti] and tungsten contents being determined as shown in FIG. 5. This superalloy has a high thermal-fatigue resistance, a great high-temperature strength, particularly, a great creep rupture strength, and a good weldability. The superalloy is used to form gas turbine nozzles, which are employed in a gas turbine. Using such a gas turbine, a combined power generating system is built.