A fuel cell has an anode (3) and a cathode (5) on either side of an electrolytic membrane (1) made of a solid polymer. A fuel supply plate (7) and an air supply plate (9) are provided at either side of the electrolytic membrane (1), so as to form a fuel cell with a solid polymer membrane. A fuel supply path (11) is provided on the anode (3) side of the fuel supply plate (7), and an air supply path (13) is provided on the cathode (5) side of the air supply plate (9). Moisture supply paths (21) are provided between fuel supply paths (11) of the fuel supply plate (7), these moisture supply paths supplying moisture directly to the electrolytic membrane (1) via the anode (3). A moisture pressure-adjusting valve, which adjusts the amount of supply of moisture by opening and closing in response to the temperature of coolant effluent from the fuel cell, is provided at a moisture introduction part which introduces moisture to the moisture supply paths (21).