A fuel cell system includes an electricity generator for generating electric energy through a reaction of hydrogen and oxygen, a reformer for generating a reforming gas containing the hydrogen from a fuel through a catalytic reaction using thermal energy and for supplying the reforming gas to the electricity generator, a burner for generating the thermal energy and for supplying the thermal energy to the reformer, a fuel supply unit for supplying the fuel to the reformer, and an air supply unit for supplying air to the electricity generator. The fuel supply unit includes a main fuel tank for storing the fuel, at least one auxiliary fuel tank connected with the main fuel tank and the reformer, and a heating unit disposed in connection with the auxiliary fuel tank to heat the fuel stored in the auxiliary fuel tank.