A fuel cell power generating apparatus using a fuel cell (10) having a structure such that a cathode and an anode are disposed on opposite sides of a polymer electrolyte membrane, comprising: an air introduction passage (11) for introducing air to the cathode; an air discharge passage (15) for discharging, to the outside of the system, discharge gas containing reactant water generated at the cathode; a circulation passage (16) connected between the air discharge passage and the air introduction passage in order to again introduce at least a portion of the discharge gas flowing through the air discharge passage to the air introduction passage and thus to the cathode; a valve (17) provided for at least one of the air introduction passage and the air discharge passage; a temperature sensor (22) for measuring the temperature of the discharge gas from the cathode; an ampere meter (19) for measuring the level of an electric current output from the fuel cell; and a control unit (18) for controlling the degree of opening of the valve in accordance with the temperature of the discharge gas and the output level of the electric current measured respectively by the temperature sensor and the ampere meter. A portion of the discharge gas containing reactant water is circulated into the air introduction passage and thus to the cathode of the fuel cell to thereby humidify the electrolyte membrane, so that there is no need to supply water from the outside of the system. The apparatus is particularly suitable to be mounted on a vehicle.