A gas turbine, in which, in operation, combustion gas that is generated by supplying fuel to a combustor where the fuel is combusted with compressed air compressed at a compressor is supplied to a turbine to acquire rotational power, the gas turbine comprising: pressurizing means connected to a branching channel branching from a discharge-side channel of the compressor and capable of carrying out an operation for introducing and pressurizing a temperature-control medium independently from the compressor; a temperature-control-medium supply channel arranged to guide pressurized temperature-control medium pressurized at the pressuring means to a turbine-cooling-medium channel (31a,50) which is provided in a blade ring, which blade ring is a stationary component of the turbine and is attached to a turbine cylinder in such a manner that it surrounds the outer circumference of rotor blades of the turbine, and which is a channel connecting the turbine cylinder, stator blades and the blade ring of the turbine and is used for temperature control by cooling and heating; a temperature-control-medium return channel arranged to guide the pressurized temperature-control medium that has passed through the turbine-cooling-medium channel (31a,50) to the discharge-side channel such that the flows are combined; and a ventilation cooling system including the pressurizing means arranged to be selectively operated at shutdown of the gas turbine to exhaust high-temperature gas remaining in the turbine or to circulate pressurized temperature-control medium through the temperature-control-medium supply channel, the turbine-cooling-medium channel (31a,50) provided in the blade ring, and the temperature-control-medium return channel. pressurizing means connected to a branching channel branching from a discharge-side channel of the compressor and capable of carrying out an operation for introducing and pressurizing a temperature-control medium independently from the compressor; a temperature-control-medium supply channel arranged to guide pressurized temperature-control medium pressurized at the pressuring means to a turbine-cooling-medium channel (31a,50) which is provided in a blade ring, which blade ring is a stationary component of the turbine and is attached to a turbine cylinder in such a manner that it surrounds the outer circumference of rotor blades of the turbine, and which is a channel connecting the turbine cylinder, stator blades and the blade ring of the turbine and is used for temperature control by cooling and heating; a temperature-control-medium return channel arranged to guide the pressurized temperature-control medium that has passed through the turbine-cooling-medium channel (31a,50) to the discharge-side channel such that the flows are combined; and a ventilation cooling system including the pressurizing means arranged to be selectively operated at shutdown of the gas turbine to exhaust high-temperature gas remaining in the turbine or to circulate pressurized temperature-control medium through the temperature-control-medium supply channel, the turbine-cooling-medium channel (31a,50) provided in the blade ring, and the temperature-control-medium return channel.