Gas turbine steam cooled system is constructed such that high temperature portion outlet steam temperature is maintained to predetermined value without cooling steam pressure becoming lower than pressure in gas turbine cylinder. Steam coming from intermediate pressure superheater 10 of waste heat recovery boiler 2 is led into high temperature portion 8 of gas turbine for cooling thereof . Temperature of cooling steam at outlet of the high temperature portion 8 is detected by high temperature portion outlet steam temperature detector 15 and signal thereof is sent to lower value selector 19 via temperature controller 11. Pressure in gas turbine cylinder is detected by gas turbine cylinder pressure detector 17 and pressure of the cooling steam at the high temperature portion 8 outlet is detected by high temperature portion outlet steam pressure detector 16 and differential signal thereof is sent to pressure controller 18. Signal from the temperature controller 11 and that from the pressure controller 18 are inputted into the lower value selector 19 and temperature control valve 12 is controlled by the lower value thereof to be opened and closed.
