Abstract: When at least either one of the output voltage and internal resistance of a plurality of stacked unit cells and the humidity of oxidizing gas being discharged has deviated from the tolerance, the supply conditions of oxidizing gas to the polyelectrolytic fuel cell are changed. Specifically, when output voltage had decreased, when internal resistance has increased, or when the humidity of discharge gas has increased, the flow rate of oxidizing gas is increased, or its pressure increases, or the humidity of oxidizing gas is reduced. As a result, moisture inside the cell is removed and flooding and the resulting falls in output voltage are prevented. In addition, the generation of hydrogen gas in the passages for oxidizing gas is contained.