A combined cycle power plant comprises a gas turbine system and a steam cycle system having a steam turbine to be driven by the steam generated by the waste heat of the exhaust gas of the gas turbine system, wherein the steam from the steam cycle system flows through a gas turbine cooling system of the gas turbine to cool the gas turbine blades and other elements of the gas turbine system to be cooled and the waste heat of the gas turbine system is effectively collected. The gas turbine cooling system is supplied selectively with bleed air from the compressor of the gas turbine or the steam provided from another steam cycle system in order to ensure a sufficient coolant supply at the time of starting and stopping the plant and during the operation with a partial load.