A steam turbine power plant which is provided with an extra-high-pressure turbine 100, a high-pressure turbine 200, an intermediate-pressure turbine 300 and a low-pressure turbine 400, and has high-temperature steam of 650.degree. C. or more introduced into the extra-high-pressure turbine 100, wherein the extra-high-pressure turbine 100 has an outer casing cooling unit which cools an outer casing 111, and a turbine rotor 112, an inner casing 110 and a nozzle box 115 of the extra-high-pressure turbine 100 are formed of an Ni base heat-resisting alloy, and the outer casing 111 is formed of a ferrite-based alloy.