NOVELTY - The fuel cell system comprises a fuel cell (40), a secondary cell (20), a converter (30) that controls output voltage of the fuel cell, a high-potential avoidance control portion that restrains the output voltage of the fuel cell from becoming greater than or equal to a predetermined heightened potential avoidance threshold voltage that is lower than an open-circuit voltage of the fuel cell by controlling a command value provided for the converter during an intermittent operation of the fuel cell, and an output detection portion that detects output current of the fuel cell.    USE - The fuel cell system is useful for a fuel cell hybrid vehicle, an electric motor vehicle and a hybrid motor vehicle, and for mobile units such as two-wheel vehicles, ships and boats, airplanes and robots.    ADVANTAGE - The fuel cell system is portable, increases the endurance of the fuel cell, and prevents overcharge of a secondary cell during an intermittent operation of the fuel cell.    DETAILED DESCRIPTION - The fuel cell system comprises a fuel cell (40), a secondary cell (20), a converter (30) that controls output voltage of the fuel cell, a high-potential avoidance control portion that restrains the output voltage of the fuel cell from becoming greater than or equal to a predetermined heightened potential avoidance threshold voltage that is lower than an open-circuit voltage of the fuel cell by controlling a command value provided for the converter during an intermittent operation of the fuel cell, and an output detection portion that detects output current of the fuel cell or the output voltage of the fuel cell. The high-potential avoidance control portion includes a converter command value correction portion that corrects the command value for the converter so that a deviation obtained by subtracting a generated power of the fuel cell from a system's allowable power that is allowable in the fuel cell system becomes equal to a value of 0 when the deviation is below the value of 0. The converter command value correction portion obtains the system's allowable power by summing a demanded power of the fuel cell system and a charging power restriction value regarding the secondary cell, and includes: a feedback control portion that performs a feedback control based on the deviation; a determination portion that determines which one of two sections of a current-voltage curve of the fuel cell divided by a bend point at which the curvature reaches a maximum value contains a point of the output current or the output voltage detected by the output detection portion; and a gain switching portion that switches gain of the feedback control based on a result of determination of the determination portion. An INDEPENDENT CLAIM is included for a control method for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a fuel cell system.    Control device (10)    Threshold voltage (11)    Converter command voltage output portion (12)    Secondary cell (20)    Converter (30)    Fuel cell. (40)