A fuel cell system is provided with a fuel cell, a system power source, an inverter system and a fuel gas supply device for supplying the fuel cell with fuel gas of a regulated quantity, wherein when the system power source falls in a power failure during the power use by the external loads, at least one of the power use by the internal loads (resistances) and the supply quantity of the fuel gas supply device is altered in dependence on the output power from the fuel cell or the power use by the external loads.