In the gas turbine combined plant, water fed to an entrance of a high-pressure economizer is guided into an air cooler as a cooling medium of high-temperature air that passes through the air cooler. High-temperature water obtained by heat exchanging with high-temperature air is branched to a passage that joins water drawn from an exit of the high-pressure economizer and a passage that joins a condenser. These passages are provided with regulating valves that regulate the flow rate of the water that passes through these passages, respectively.