A fuel cell system employs a fuel cell stack having an air electrode, an electrolyte electrode, and a fuel electrode. While the fuel cell system is operated, air is supplied to an air electrode and hydrogen is supplied to a fuel electrode.
However, if air stays behind in the interior of the system after the system is stopped, carbon corrosion due to oxygen in the air may occur. This may lower the durability of the fuel cell stack, and thus is not desirable. Further, if too much hydrogen stays behind in the interior of the system after the system is stopped, a concentration of hydrogen increases too much in the gases discharged when the system is restarted and thus a danger of a fire or an explosion may increase, which is undesirable.