Moisture caused by humidity in the fuel gas and water vapor from the water that is generated become condensed inside a fuel cell when power generation in the fuel cell is temporarily stopped, making obstruction to the fuel gas flow channel when power generation is restarted possible. A fuel cell includes an electrolyte membrane sandwiched between a fuel electrode and an oxidant electrode. Oxidant is supplied to the oxidant electrode in the fuel cell, and the fuel emitted from the fuel electrode of the fuel cell is resupplied back to the fuel electrode. When requested power generating capacity for the fuel cell is less than a prescribed capacity, the oxidant supply is temporarily stopped while the fuel continues to circulate in order to prevent obstruction in the fuel flow channel due to water condensation, making a reliable fuel supply becomes possible.