An advanced humid air turbine power plant capable of suppressing condensation of moisture in a cooling channel, which is provided to cool a high-temperature component of a gas turbine, in a start up stage, a coast down stage, and a load varying state of the gas turbine. In the advanced humid air turbine power plant comprising a compressed air supply line for supplying compressed air generated by a compressor to the high-temperature component of the gas turbine, and a humidified air supply line for supplying humidified air generated in a humidifying tower to the high-temperature component of the gas turbine, the power plant further comprises valves capable of adjusting respective air flow rates in the compressed air supply line and the humidified air supply line. A coolant is switched over from the compressed air to the humidified air in the start up stage and the load increasing stage of the gas turbine, and from the humidified air to the compressed air in the load decreasing stage and the coast down stage, as well as in the event of an abnormality in a humidifying line.