The fuel cell system according to this invention disposes a moisture exchanger (30) at a suction side of an air supply device (7). This causes an increase in the pressure differential and temperature differential of the fresh air and the cathode exhaust in the moisture exchanger (30) and promotes transfer of moisture. Since the moisture exchanger (30) effectively transfers moisture, it is possible to downsize the radiator for the cooling water and the condenser (60). Thus it is possible to reduce the weight and size of the fuel cell system.