Water is flowed down into heat transfer tubes from their inlet ports to form flowing water films. The flowing water is evaporated from the flowing water films, thereby cooling combustion air of a gas turbine that flows in the channel. Water vapor generated in the heat transfer tubes is absorbed by an absorber by using an aqueous lithium bromide solution. Water vapor contained in the aqueous solution supplied from the absorber is extracted. The extracted water vapor is condensed to water by a condenser through which cooling water flows. The obtained water is supplied to the water tank. A condensed aqueous solution from which the water vapor has been removed by the generator is supplied to the absorber.