Abstract: A combined power plant, which has, as one of lines for supplying gas turbine cooling duty steam for cooling hot parts of a gas turbine, a line branching off from a steam line for leading high pressure steam from a high pressure superheater of a multi-pressure heat recovery steam generator to a high pressure steam turbine and leading a part of the high pressure steam to a supply line of the gas turbine cooling duty steam. The combined power plant is provided with a temperature lower limit value setting means for setting a lower limit value of an allowable temperature range for each gas turbine load and supplies a part of hot high temperature steam to the hot part of the gas turbine when the gas turbine cooling duty steam temperature is lower than the lower limit value of the allowable temperature range, thereby to raise the gas turbine cooling duty steam temperature.