PURPOSE: To improve the plant efficiency by feeding fuel to keep the temperature of the exhaust gas at the preset temperature or above, and maintaining the revolving speed at the preset speed.

CONSTITUTION: The temperature of the exhaust gas discharged from a turbine 4 is detected at the partial load of a system 9, and fuel is fed to an auxiliary combustor 11 to keep it at the preset temperature or above. The fuel consumption of the combustor 11 can be suppressed to the minimum while the temperature of the steam generated by an exhaust heat boiler 14 is kept within the desired range. The turbine 4 is driven by a generator-motor 6 to keep a compressor at the preset revolving speed when the power of the turbine 4 is low in particular, thus the fuel consumption can be reduced. The plant efficiency can be increased even when a part of the electricity generated by the power generating system 9 is used for the motor 6. The auxiliary fuel consumption is reduced, the generated quantity of NOx is reduced, and the environmental pollution characteristic can be kept low.