Fuel cell system, a method of estimating the anode gas generation amount estimation apparatus and the anode gas generation amount

Power generation efficiency to provide such possible fuel cell system can be sufficiently reduced exhaust hydrogen concentration even if the fuel cell is operated at a low state. Providing a bypass valve B1 between the oxidizing gas oxidizing gas supply path 11 cathode off gas channel 12. Since the supply of the oxidizing gas to the cathode is in a state of insufficient include pumping hydrogen into the cathode offgas, by adjusting the valve opening degree of the bypass valve B1, controls the exhaust hydrogen concentration by adjusting the flow rate of the bypass air.