When power failure of a commercial power source is detected, an inverter interrupts power supply to a power line extending between the commercial power source and a load via a circuit breaker. Then the control mode of a load DC—DC converter is changed to a constant voltage control to maintain stable operation of a fuel cell. This allows the accessory DC—DC converter to supply power to accessories used for operating the fuel cell power generation system. Also, the quantity of city gas to be supplied to a reformer is changed to a quantity necessary for the fuel cell to generate power for driving the accessories. This makes it possible to avoid excessive production of hydrogen, thus improving energy efficiency of the fuel cell power generation system.