NOVELTY - The method involves implementing regeneration-time voltage fixed control where during regeneration or when regeneration is expected to occur. The occurrence state of regeneration is determined in the regeneration-time voltage fixed control based on movement of moving object such as fuel cell vehicle (10) with or without fuel cell system (12) down a slope. The amount of reactant gas supplied to fuel cell is decreased by determining occurrence of regeneration while the moving object is not moving down a slope.    USE - Method of controlling fuel cell system for fuel cell vehicle. Can also be used in movable objects such as electric train or electric bicycle.    ADVANTAGE - The degradation of fuel cells can be suppressed, while effectively collecting regenerative electric power, since output voltage of fuel cell is set to a voltage value outside the oxidation reduction progress voltage range where oxidation reduction proceeds. Since the regenerative electric power with which the energy storage device is charged is increased, the amount of electric power remaining in the energy storage device can be made closer to the target value easily, or the amount of electric power remaining in the energy storage device can be maintained within the target range easily, thus the regeneration efficiency is improved and the power generation efficiency of the fuel cell system is improved.    DETAILED DESCRIPTION - The output voltage of a fuel cell is fixed to a voltage value outside an oxidation reduction progress voltage range where oxidation reduction proceeds. The amount of a reactant gas supplied to fuel cell is changed based on amount of electric power remaining in energy storage device.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the overall structure of fuel cell vehicle equipped with fuel cell system. (Drawing includes non-English language text)    Fuel cell vehicle (10)    Fuel cell system (12)    Traction motor (14)    Inverter (16)    Fuel cell unit (18)