Abstract: A fuel cell system 10 has a fuel cell 16 and a battery 24. An external load 30 is supplied with output power produced by the fuel cell and output power discharged from the battery. The fuel cell system includes a battery charge level monitoring unit 26 which monitors and detects a battery charge level, an accelerator' displacement sensor 32 which detects an accelerator's displacement value and a control unit 11 which calculates demand power to be produced by the fuel cell and varying rate of the displacement value. The control unit corrects the demand power by using a control map having a power correction pattern that is in response to a varying rate of the displacement value and the charge level of the battery.