A fuel cell power generating apparatus includes a stack of a plurality of fuel cell units each having a cathode and an anode disposed on opposite sides of an electrolyte membrane. A fuel gas supply system supplies fuel gas to the anode, an air supply system supplies air to the cathode and a water supply system supplies liquid water to the cathode. A control unit ensures that, when the apparatus starts up, the cathode is first supplied with air, followed by supply of the liquid water. In a preferred embodiment, the water supply system intermittently sprays the liquid water onto the cathode when the temperature of the fuel cell stack monitored by a temperature sensor falls below a predetermined temperature.