NOVELTY - The method involves performing low electric-power generation prior to oxygen-consumption electric-power generation, where a charge state in an electricity storage device in a fuel cell system is larger than a predetermined value when a system stop switch (31) is operated. The oxygen-consumption electric-power generation in which oxygen remaining in a cathode system of a fuel cell (1) is consumed to reduce oxygen concentration within the cathode system is performed after the charge state is reduced to the value by compensating a negative net output by discharging electricity from the device.    USE - Method for deactivating a fuel cell system used in a fuel cell vehicle.    ADVANTAGE - The method enables time-effectively performing the oxygen-consumption electric-power generation based on variable electric-power generation if the charge state in the device is larger than or equal to the value such that the oxygen-consumption electric-power generation can be reliably performed, while adjusting the charge state in the device and balancing the charge state in an electricity storage unit, the charge state does not exceed an upper limit value, and oxygen concentration within the cathode system of the fuel cell can be sufficiently reduced upon completion of the oxygen-consumption electric-power generation, so that the oxygen-consumption electric-power generation in the fuel cell can be stopped in a state, where the oxygen concentration within the cathode system is sufficiently reduced, and the cathode system can be reliably prevented from being in a high potential state at time of subsequent activation of the fuel cell system, thus preventing deterioration of a solid polymer electrolyte membrane of the fuel cell, and eliminating variations in an output of the fuel cell and disturbance during the oxygen-consumption electric-power generation.    DETAILED DESCRIPTION - The device is a high-voltage battery (41). An INDEPENDENT CLAIM is also included for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a fuel cell system that is deactivated.    Fuel cell (1)    Fuel flow passage (5)    Hydrogen tank (20)    Regulator (24)    System stop switch (31)    High-voltage battery (41)