PURPOSE: To enhance the cooling effect of turbine blades, by cooling air efficiently by injecting water and utilizing compressed air as primary air only when the temperature of compressed air used for cooling the turbine blades is higher than a prescribed value.

CONSTITUTION: A gas turbine 1 is composed of a compressor 2, a combustor 3, a turbine 4, etc., and a temperature detector 6 and water injection means 5 are provided in a cooling-air chamber 12 defined in a combustor chamber 11. The water injection means 5 is connected o a water supply source 9 disposed on the outside of the gas turbine 1 via a pipe 8, and a valve 7 disposed at an intermediate portion of the pipe 8 is controlled by a controller 10. Further, primary air is supplied from a high-pressure air source 34 to the water injection means 5 via a pipe 33, and a valve 32 disposed at an intermediate portion of the pipe 33 is also controlled by the controller 10. With such an arrangement, it is enabled to cool air efficiently by opening the valves 32, 7 sequentially when the temperature of air in the cooling-air chamber 12 is higher than a prescribed value.