PURPOSE: To discharge excessive water in a fuel cell from gas passages without causing the leakage of gases and infiltration of outside air.

CONSTITUTION: In a fuel cell system, in which oxidizing gas and fuel gas are supplied to both sides sandwiching a solid electrolyte film while at least either one of the gases is humidified and in which an oxidation reaction that generates reaction products including water is effected via the solid electrolyte film, a first liquid storage portion 6 which branches downward from passages 2, 4 through which either of the gases circulates or from another passage communicated with these passage is provided. A second liquid storage portion 7 is connected to the lower part of the first liquid storage portion 6, and valves 9, 10 which are alternately opened are provided between the liquid storage portions 6, 7 and on the outlet side of the second liquid storage portion 7.