To perform efficient cooling of intake air.

In a heat-exchanger to cool intake air, intake air is cooled by an intake air cooling subsystem and a cooling subsystem using the heat of vaporization of LNG and the latter is used for cooling of a part where temperature is low. Further, purified mist (fog) is mixed in intake air of a gas turbine and a substantial cooling efficiency is increased. In the intake air cooling subsystem, when ammonia is used as a refrigerant and water is used as an absorbing, high concentration ammonia steam vaporized by absorbing the heat of intake air is absorbed in a low concentration ammonia solution remained after vaporization of ammonia, and the absorbing heat is used in vaporization of the refrigerant. Further, in this case, in a heat-exchanger to cool the intake air, vaporization of the refrigerant is effected by a pressure in a plurality of stages and an amount (a ratio) of the refrigerant used in vaporization cooling is increased (ensured).