The capacity reduction of a solid-polymer-membrane-type fuel cell (PEFC) with a reformer by load leveling and by improving the efficiency of part-load operation of the reformer is considered. The power generation efficiency of a fuel cell improves by supplying gas with a high oxygen concentration to the cathode. During periods of low electricity demand, the fuel cell is operated using reformed gas and air, and water electrolysis operation is also performed. When the electric power load is large, gases stored in cylinders are supplied to the fuel cell for operation. Using the proposed method, high efficiency operation and a reduction in the fuel cell capacity are possible.