A fuel cell system which can encase a dilution device while keeping the height of a fuel cell case as low as possible by utilizing the lower space in the case effectively. A fuel cell system comprises a fuel cell stack generating power through an electrochemical reaction between a gas supplied to the anode side and a gas supplied to a cathode side, a dilution device for diluting an anode off gas discharged from the fuel cell stack with a cathode off gas and discharging the diluted gas, and a fuel cell case for encasing the fuel cell stack and the dilution device. In this fuel cell system, a lateral opening of the fuel cell case for passing an exhaust pipe extending to the exhaust downstream of the dilution device is arranged above the lowermost portion of the inner surface of the dilution device with respect to the gravitational direction.