To improve plant thermal efficiency by cooling a part of air compressed by a compressor in a cooling air type cooler, supplying its obtained cooled air to a device which is thermally loaded, evaporating water supplied to cooling, and guiding its steam in a process.

A gas turbo device is provided with a compressor 1, and a high-pressure combustion chamber 2, a high-pressure turbine 3, a low-pressure combustion chamber 4, and a low-pressure turbine 5 are arranged downstream in sequence. Air 8 compressed by a compressor 1 is guided in the high-pressure combustion chamber 2 and is burnt with fuel 9 in the combustion chamber 2. Generated high-pressure and high-temperature gas are introduced into the high-pressure turbine 3, the low-pressure combustion chamber 4, and the low- pressure turbine 5 in sequence, and thereby, a power generator 6 is driven. A part of air compressed by the compressor 1 is supplied to a desired position as cooled air 19, after it is cooled by a cooling air cooler 16, and water flow 20 is converted into steam 21 in the cooler 16 to guide into a process.