PURPOSE:To set the load fluctuation range to about 4-100% which is larger than before by controlling the fuel pressure in response to the change of the electric output. CONSTITUTION:The heat quantity fed to an expansion turbine 37 is controlled in response to the fuel pressure, the discharge air pressure of a compressor 36 is controlled, thereby the fuel pressure can be changed while the pressure difference between the anode 9 and the cathode 11 of a fuel cell 8 is suppressed within the allowable value. The power for the pressure rise is decreased by the amount that the air feed pressure is reduced, and the efficiency of the partial load is increased. When the discharge air pressure of the compressor is changed, the operation range of an expansion turbine 37 and the compressor 36 is increased, and the operation property at the partial load is improved. When fuel is fed to an auxiliary combustion device 69 at the inlet of an exhaust heat recovery heat exchanger or the inlet of the expansion turbine 37 independently of the operation restriction of the electric output, the electric/heat output ratio can be optionally selected, thus both the electric power demand and the heat demand can be coped with.