A steam turbine plant partially utilizes a hydrocarbon fuel such as an LNG as a fuel to improve the efficiency of the plant. A steam turbine plant includes a boiler, a steam turbine, a feedwater system, and a superheating combustor. The boiler generates steam, and the steam turbine is driven by the steam generated in the boiler. The feedwater system recovers the steam exhausted from the steam turbine and supplies the steam to the boiler as a feedwater. The superheating combustor is provided between the boiler and the steam turbine, wherein fuel originated from hydrocarbon fuel and oxidizer are mixed with the steam generated in the boiler and combusted in the superheating combustor to increase the temperature of the steam.