NOVELTY - The method involves determining whether temperature of fuel cell stack (1001) is higher than initial working temperature, and entering system (100) stable mode if any condition is positive. The current output (Iout) by stack via voltage regulating circuit is determined whether smaller than minimum output current or not. The working voltage (V1) of auxiliary cell (1005) is determined whether greater than charging set value. The stack is operated in stable mode if all conditions are positive. The system is exited from system stable mode if any condition is negative.    USE - Method for controlling fuel system.    ADVANTAGE - Since the current output by fuel cell stack through voltage regulating circuit is determined whether smaller than minimum output current or not, the power supply demands of the load are reduced effectively while the output current of the system is smaller than the threshold. The working voltage is determined as greater than charging set value when the current output is smaller than the minimum output current. Thus the fuel cell system is entered stand-by mode, and the stand-by mode is maintained efficiently when any condition is unsatisfied. The initial working temperature of the fuel cell stack is set as lowest temperature at which the electrochemical reaction is performed in the fuel cell stack and current can be stably outputted. The stand-by mode is maintained efficiently when the counted time of the CPU timer is shorter than the time set for entering the sleep mode. The importance of the management of the power energy sources is increased efficiently, and the fuel cell system is controlled efficiently. Thus the performance, reliability, and the service life of the system are enhanced efficiently.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for fuel system.    DESCRIPTION OF DRAWING(S) - The drawing shows a circuit diagram of the fuel cell system.    Fuel cell system (100)    Fuel cell stack (1001)    Auxiliary cell (1005)    Current output (Iout)    Working voltage (V1)