A gas turbine comprises: a first closed circuit in which cooling gas is supplied to a casing, allowed to pass through stationary blades, and removed to the outside of casing after cooling the stationary blades; a second closed circuit in which cooling air is supplied to the casing, allowed to pass through moving blades, and removed to the outside of casing after cooling the moving blades; a cooler for cooling the cooling gas in the first and second closed circuits; and a booster for pressurizing and circulating the cooling gas cooled by the cooler.Also, A gas turbine comprises: a first closed circuit in which cooling air cooled after being extracted from a combustor plenum is allowed to pass through stationary blades successively to cool the stationary blades, and then recovered and returned to a compressor on the upstream side of the combustor plenum; and a second closed circuit in which cooling air cooled after being extracted from the combustor plenum is allowed to pass through moving blades successively to cool the moving blades, and then recovered and returned to the compressor.