NOVELTY - Controlling a fuel cell system involves detecting a temperature of a fuel cell stack (10); detecting a temperature of reformate that is generated in a fuel reformer (20) and then is supplied to the fuel cell stack through a heat exchanger (30); and setting the temperature of the reformate to be lower than the temperature of the fuel cell stack during a starting time of the fuel cell system.    USE - For controlling a fuel cell system (claimed).    ADVANTAGE - The control method is capable of preventing anode flooding due to a temperature difference between a stack and reformate upon starting a fuel cell system. The method supplies the reformate with lower temperature than the stack temperature in the predetermined range upon starting the system, thus makes it possible to prevent the anode flooding problem caused in the inside of the existing stack. The stack temperature is increased in the starting period, and the reformate temperature is increased thus makes it possible to increase the starting and operation stability of the stack and prevent the degradation of the stack performance. The stack temperature is raised exceeding the set temperature after starting the system, the reformate temperature is linked with the stack temperature, thus makes it possible to prevent the inside of the stack to be dried.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a fuel cell system comprising a fuel cell stack for producing electricity; a first sensor detecting a temperature of the stack; a fuel reformer generating reformate and supplying the reformate to the fuel cell stack; a heat exchanger coupled between the stack and the reformer; a second sensor detecting the temperature of the reformate in the heat exchanger; and a controller coupled to each of the stack and the heat exchanger. The fuel cell system further comprises an oxidant supplier supplying an oxidant to a cathode of the stack; the stack comprises a stack in a polymer electrolyte membrane fuel cell scheme.    DESCRIPTION OF DRAWING(S) - The figure shows block diagram of fuel cell system.    Fuel cell stack (10)    Fuel reformer (20)    Heat exchanger (30)    First sensor (42)    Controller (50)