A gas turbine having a compressor for compressing air, a combustor for taking in compressed air discharged from the compressor, mixing it with fuel, and burning them, and a turbine driven by combustion gas generated in the combustor, comprising: inlet guide vanes installed at a stage near an inlet of the compressor for adjusting a compressor flow rate by changing an attaching angle thereof, a steam injection mechanism for injecting steam to the combustor, a steam adjustment valve for adjusting the steam injection rate, and a steam rate control mechanism for adjusting an opening of the steam adjustment valve, a steam monitoring mechanism for monitoring the steam rate injected to the combustor, an air temperature monitoring mechanism for monitoring an atmospheric temperature, and a control unit for determining whether a restriction value of the steam injection rate using a temperature, an opening of the inlet guide vanes, and the steam injection rate as indexes is satisfied, and controlling at least one of a temperature of air flowing into the compressor, the steam injection rate, and the inlet guide vane opening.