A steam turbine power plant comprising a high-pressure turbine, an intermediate-pressure turbine and a low-pressure turbine, the intermediate-pressure turbine being separated into a high-temperature, intermediate-pressure turbine section (11a, 70, 80, 90) into which high-temperature steam having steam exhausted from the high-pressure turbine reheated to 650°C or more is introduced and a low-temperature, intermediate-pressure turbine section into which steam exhausted from the high-temperature, intermediate-pressure turbine section (11a, 70, 80, 90) is introduced, wherein a steam cooling unit for cooling a rotor of the high-temperature, intermediate-pressure turbine section (11a, 70, 80, 90) by cooling steam is provided, at least one composing element of the high-pressure turbine, the low-temperature, intermediate-pressure turbine section and the low-pressure turbine is formed of a ferritic alloy steel, and the rotor of the high-temperature, intermediate-pressure turbine section (11a, 70, 80, 90) is formed of, on percentage by weight basis, one alloy steel selected from a group consisting of 10) to 14): 10) an alloy steel consisting of C: 0.08 to 0.15, Si: 0.1 or less, Mn: 0.1 to 0.3, Ni: 0.1 to 0.3, Cr: 9.0 to less than 10.0, V: 0.15 to 0.3, Mo: 0.6 to 1.0, W: 1.5 to 2.0, Co: 1.0 to 4.0, Nb: 0.02 to 0.08, B: 0.001 to 0.015, N: 0.01 to 0.06 and the balance of Fe and unavoidable impurities and having M 23 C 6 type carbide precipitated mainly on grain boundaries and martensitic lath boundaries by a tempering heat treatment, M 2 X type carbonitride and MX type carbonitride precipitated within the martensitic lath, a relationship of V>Mo between V and Mo in the component elements of the M 2 X type carbonitride, and a total of precipitates of the M 23 C 6 type carbide, the M 2 X type carbonitride and the MX type carbonitride being 2.0 to 4.0% by weight; 11) an alloy steel consisting of C: 0.08 to 0.15, Si: 0.1 or less, Mn: 0.1 to 0.8, Ni: 0.1 to 0.8, Cr: 9.0 to less than 11.0, V: 0.15 to 0.3, Mo: 0.8 to 1.4, Nb: 0.02 to 0.3, N: 0.01 to 0.06 and the balance of Fe and unavoidable impurities; 12) an alloy steel consisting of C: 0.08 to 0.15, Si: 0.1 or less, Mn: 0.1 to 0.8, Ni: 0.1 to 0.8, Cr: 9.0 to less than 11.0, V: 0.15 to 0.3, Mo: 0.8 to 1.4, Nb: 0.02 to 0.3, N: 0.01 to 0.06, W: 0.5 to 1.4 and the balance of Fe and unavoidable impurities; 13) an alloy steel consisting of C: 0.13 to 0.35, Si: 0.2 or less, Mn: 0.8 or less, Ni: 0.8 or less, Cr: 0.8 to 1.9, V: 0.2 to 0.35, Ti: 0.01 or less, Mo: 0.7 to 1.4 and the balance of Fe and unavoidable impurities; and 14) an alloy steel consisting of C: 0.13 to 0.35, Si: 0.2 or less, Mn: 0.8 or less, Ni: 0.8 or less, Cr: 0.8 to 1.9, V: 0.2 to 0.35, Ti: 0.01 or less, Mo: 0.7 to 1.4, W: 0.8 to 1.4 and the balance of Fe and unavoidable impurities. 10) an alloy steel consisting of C: 0.08 to 0.15, Si: 0.1 or less, Mn: 0.1 to 0.3, Ni: 0.1 to 0.3, Cr: 9.0 to less than 10.0, V: 0.15 to 0.3, Mo: 0.6 to 1.0, W: 1.5 to 2.0, Co: 1.0 to 4.0, Nb: 0.02 to 0.08, B: 0.001 to 0.015, N: 0.01 to 0.06 and the balance of Fe and unavoidable impurities and having M 23 C 6 type carbide precipitated mainly on grain boundaries and martensitic lath boundaries by a tempering heat treatment, M 2 X type carbonitride and MX type carbonitride precipitated within the martensitic lath, a relationship of V>Mo between V and Mo in the component elements of the M 2 X type carbonitride, and a total of precipitates of the M 23 C 6 type carbide, the M 2 X type carbonitride and the MX type carbonitride being 2.0 to 4.0% by weight; 11) an alloy steel consisting of C: 0.08 to 0.15, Si: 0.1 or less, Mn: 0.1 to 0.8, Ni: 0.1 to 0.8, Cr: 9.0 to less than 11.0, V: 0.15 to 0.3, Mo: 0.8 to 1.4, Nb: 0.02 to 0.3, N: 0.01 to 0.06 and the balance of Fe and unavoidable impurities; 12) an alloy steel consisting of C: 0.08 to 0.15, Si: 0.1 or less, Mn: 0.1 to 0.8, Ni: 0.1 to 0.8, Cr: 9.0 to less than 11.0, V: 0.15 to 0.3, Mo: 0.8 to 1.4, Nb: 0.02 to 0.3, N: 0.01 to 0.06, W: 0.5 to 1.4 and the balance of Fe and unavoidable impurities; 13) an alloy steel consisting of C: 0.13 to 0.35, Si: 0.2 or less, Mn: 0.8 or less, Ni: 0.8 or less, Cr: 0.8 to 1.9, V: 0.2 to 0.35, Ti: 0.01 or less, Mo: 0.7 to 1.4 and the balance of Fe and unavoidable impurities; and 14) an alloy steel consisting of C: 0.13 to 0.35, Si: 0.2 or less, Mn: 0.8 or less, Ni: 0.8 or less, Cr: 0.8 to 1.9, V: 0.2 to 0.35, Ti: 0.01 or less, Mo: 0.7 to 1.4, W: 0.8 to 1.4 and the balance of Fe and unavoidable impurities.