A fuel cell device, wherein, after an output voltage from a main body of a fuel cell is converted using a converter, the relation between the resultant predetermined output voltage V 1 and an output voltage V 2 from a secondary battery is so set as to satisfy V 1>V 2. When, at the time of a sudden change of an external load, the output voltage V from the main body of the fuel cell becomes lower than a predetermined voltage V 3, an output to a charge controlling unit is stopped. When the output voltage V from the main body of the fuel cell is lowered even further and becomes lower than a predetermined voltage V 4, an output to an auxiliary device, which is necessary for driving the fuel cell device, is switched from the output from the converter to the output from the secondary battery.