NOVELTY - The method involves stopping supply of fuel gas to a fuel cell (20), supplying oxide gas to the cell to generate power from an oxide gas supply apparatus (14). Power generation of the cell is stopped when operation stop instruction to stop the operation of the cell is detected. Supply of the oxide gas from the supply apparatus after elapse of a preset time from starting of supply of the fuel gas is started, when an operation start instruction to start a fuel cell system (10) is detected and pressure of an anode side is equal to or less than threshold pressure.    USE - Method for controlling a fuel cell system of a fuel cell vehicle i.e. fuel cell car.    ADVANTAGE - The method enables start supply of the fuel gas from the fuel-gas supply apparatus before starting supply of the oxide gas from the oxide-gas supply apparatus, and starting supply of the oxide gas after elapse of the predetermined time from the starting of supply of the fuel gas upon detection of the operation start instruction to start the fuel cell system after the power generation is stopped and execution of in-stop-mode power generating process when the pressure of the anode is equal to or less than the threshold pressure, so that the fuel cell system can be started without increasing inter-electrode differential pressure between an anode and a cathode to prevent erroneous opening of an air inlet valve. The method enables increasing supply pressure of the hydrogen gas to the fuel cell stack before supply of the hydrogen gas is stopped, so that low-oxygen stoichiometric power generation is carried out properly with the fuel cell stack to be filled with adequate amount of the hydrogen, thus preventing excessive hydrogen gas from retaining inside the fuel cell stack or hydrogen gas in the fuel cell stack from running short after discharge is completed so as to limit nitrogen-gas replacement range in the fuel cell stack when the power generation of the fuel cell stack is carried out while supplying air via an air pump to expand nitrogen-gas replacement range to an inlet side of the fuel cell stack when the power generation of the fuel cell stack is carried out after the air pump is stopped, and hence preventing deterioration of fuel cells on the cathode side even when the fuel cell system is stopped for a longer period of time.    DETAILED DESCRIPTION - The fuel gas is hydrogen gas, and the oxide gas is air.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a fuel cell system.    Fuel cell system (10)    Fuel cell stack (12)    Oxide-gas supply apparatus (14)    Fuel cell (20)    Humidifier (54)