PURPOSE: To operate a fuel cell with its across-the-electrode pressure drop being controlled to be always kept at the optimum set value, by comparing a signal from a pressure drop indicator to the optimum set value and thereby making required adjustments, wherein the optimum set value of the across-the-electrode pressure drop is determined based on signals from a supplied gas flowmeter, a discharged gas flowmeter, and the load apparatus.

CONSTITUTION: Signals from a pressure drop indicator 10 for measuring the pressure difference between the fuel electrode and a reference pressure, a flowmeter 11 on the fuel supply side, a flowmeter 12 on the fuel discharge side, and the load 13 are input to an arithmetic unit 9 so that a new optimum set value for the pressure drop is determined and the actual pressure drop is adjusted to take the set value by the opening and shutting of a discharge valve 14. The same arrangement is applicable also to the oxidant electrode side and the discharge valve 19 is opened or shut likewise. Since the quantity of H2 and O2 consumed by crossover can be computed from mass balance of H2 or O2, it becomes possible to make automatic control of the pressure drop to converge into the optimum value by providing fine adjustments. The operation with the optimum set value being maintained regardress of internal conditions of the cell can thus be achieved.