A high-efficiency power generation method in which gas turbines and steam turbines are combined. City gas, mixed with compressed air, is combusted in the burner, and the pressure generated by the combustion drives a turbine to generate electricity. Next, the heat of the exhaust gas from the turbine is used to generate high-pressure steam in a waste-heat collecting boiler, and this steam drives a steam turbine to generate electricity. This use of heat from the exhaust gas gives the gas turbine combined cycle method a high power generating efficiency.