To improve an efficiency and specific output by injecting part of the steam superheated in a waste-heat steam generator at the proper place in a gas turbine group, in a power plant consisting of a gas turbine group and the waste-heat steam generator flowing an exhaust gas.

A gas turbine group having a compressor 1 is provided with a first combustion chamber 4, first turbine 7, second combustion chamber 9 and second turbine 12 continuously on the downstream side of this compressor 1 in order and a generator 19 is connected to a common rotor shaft side 24. The exhaust gas from the second turbine 12 is streamed in the waste-heat steam generator 14 and one part 15 of the steam superheated here is injected into the second combustion chamber 9. The other part of the steam is taken out from a drum 20 as a saturated steam 16 and introduced to a heat exchanger 17 and 100% of energy is collected by the superheat by the heat of the compressed air. The obtained superheated steam 25 is thrown into the combustion chamber 4 and can be utilized to the output increase of a power plant.