The reservoirs 2 and 2′ preliminarily contain liquid water, which is utilized as the water to be supplied to the polymer membrane. A vapor pressure of the water is set to a predetermined value in the reservoir by controlling the temperature of the reservoirs 2 and 2′ individually. Pressure gauges 6 and 6′ may be used for setting a vapor pressure of the water. The water which is gasified based on the set vapor pressure in the respective reservoir is supplied to the stack 10 along with oxygen from the reservoir 2, and with hydrogen from the reservoir 2′. This configuration makes it possible to adjust the amount of water contained in the polymer membrane and maintain the moisturization of the polymer membrane without external water supply.