PURPOSE: To improve the overall efficiency of the power generating system at the time of partial load operation, by operating the fuel cell then at lower operating pressure than that at the full load operation through the variable pressure and variable airflow operation of the air compressor.

CONSTITUTION: A fuel 2 is reformed with steam and denatured by a reforming apparatus 4 into hydrogen fuel and the same is supplied to the fuel cell 8. Air is also supplied to the cell through a compressor 22 equipped with a heat recovery turbine 40, and the air is adapted to react there with the hydrogen to generate electricity. At the time of partial load operation, auxiliary fuel 3 to be supplied to a burner 28 installed at the inlet line of the turbine 40 is so adjusted that the airflow measured by a flowmeter installed in the discharge line of the compressor 22 indicates such a value that corresponds to the load value, whereby the compressor 22 can be operated to provide varied pressure and airflow. Thus, it becomes possible to operate the cell 8 under varied pressure conditions provided by the compressor 22, and the improvement of the overall efficiency of the power generating system at the partial load operation can be achieved.