NOVELTY - A fuel cell system operation comprises setting temperature threshold value for detected temperature and upper-limit threshold value for detected amount; comparing detected amount and detected temperature with respective threshold value; disconnecting connection between cell and electrical load; and driving circulation pump, where fuel cell system comprises cell, buffer unit (106), liquid amount detector (111), temperature detector (101), circulation pump (107), fan, fuel supply unit (110) and secondary battery.    USE - Method for operating fuel cell system used for driving electrical load.    ADVANTAGE - The method adjusts amount of liquid stored in buffer unit, thus providing stable operation.    DETAILED DESCRIPTION - The fuel cell system operation comprises:    (A) setting a temperature threshold value for detected temperature and upper-limit threshold value for detected amount;    (B) comparing the detected amount with upper-limit threshold value;    (C) comparing detected temperature with temperature threshold value;    (D) disconnecting connection between cell and electrical load, and then driving fan with the secondary battery; and    (E) driving circulation pump to circulate fuel in route with secondary battery when the detected amount is larger than the upper-limit threshold value and the detected temperature is lower than or equal to the temperature threshold value.    The fuel cell system comprises cell comprising anode electrode and cathode electrode sandwiching an electrolyte membrane; buffer unit to store the fuel; liquid amount detector to detect an amount of the fuel stored in the buffer unit; temperature detector to detect a temperature of at least one of the cell and the fuel flowing in the cell; circulation pump to circulate the fuel in a route from the buffer unit and returning to the buffer unit through the anode electrode; fan to cool the cell; fuel supply unit to supply the fuel to the route; and secondary battery to store the power generated by the cell. The cell generates power to drive electrical load by reaction of fuel supplied to the anode electrode with air supplied to cathode electrode. An INDEPENDENT CLAIM is included for the fuel cell system comprising cell, buffer unit, liquid amount detector, temperature detector, circulation pump, fan, fuel supply unit, secondary battery, and controller to set temperature threshold value.    DESCRIPTION OF DRAWING(S) - The drawing is a block diagram showing a fuel cell system.    Temperature detector (101)    Buffer unit (106)    Circulation pump (107)    Fuel supply unit (110)    Liquid amount detector (111)