The present invention provides the fuel cell system, in which the decrease in the power generation and the degradation of the durability of the fuel cell can be prevented even if the temperature of fuel cell is low at starting or the ambient temperature is low. On the fuel cell starting electric power generation, the air supplied to the cathode inlet is humidified by the hollow fiber membrane humidifier and the hydrogen gas supplied to be anode inlet is humidified by another hollow fiber membrane humidifier. The cooling fluid of the cooling line, which absorbs the heat from the fuel cell and flows into the heat exchanger, heats the hollow fiber membrane humidifiers. Thus the humidification is promoted by heating the humidified air and hydrogen gas to raise their dew-point temperatures.