A concentration of a fuel supplied to a fuel cell and a heat exchange rate of a heat exchanger are controlled simultaneously in order to maintain a temperature of fuel cell stack and a diluted fuel concentration that is supplied to the fuel cell stack in a target level. Further, by appropriately responding to an exterior temperature changes or unexpected fuel cell performance changes, it is possible to improve a system efficiency and stability of fuel cell operation and to regulate the fuel concentration quickly and accurately compared to a prior sensor-less concentration controller.