A water tank 21 for stocking processed water used for cooling of a solid polymer type fuel cell 6, etc., cooling water supply means for supplying the processed water to the fuel cell 6 reactant air supplying means for supplying the reactant air to the fuel cell 6, and fuel gas supply means for electrode reaction are provided, and the reactant air supply means passes the reactant air through the water tank 21 to add moisture to the gas.