A superalloy having more excellent hot corrosion resistance than conventional single crystal alloys, oxidation resistance levels as high as that of conventional single crystal alloys, high strength, and creep rupture strength; single crystal members used for a blade or a nozzle produced by utilizing such an alloy; and a combined cycle power generation system produced by utilizing such members. A highly hot corrosion resistant and high-strength superalloy consisting essentially of, by weight, 6-12% Cr, 4.5-6.5% Al, 2-12% W, 2.5-10% Ta, not more than 5.8% Mo, 0.1-3% Co, 0.2-3% Nb, 0.1-4% Re, not more than 0.3% Hf, and the balance being Ni and unavoidable impurities; single crystal members used for a blade or a nozzle produced utilizing such an alloy; and a combined cycle power generation system produced by utilizing such members.