A fuel cell system including a fuel cell; a cathode air heat exchanger configured to perform heat exchange between a cathode exhaust gas and the air to be supplied to the cathode to transfer a part of heat energy of the cathode exhaust gas to the air; a desulfurization unit configured to remove a sulfur component from a raw material supplied to the desulfurization unit; and a reformer configured to generate a reformed gas which is the fuel from steam and the raw material from which the sulfur component has been removed by the desulfurization unit; and the cathode exhaust gas which has lost a part of the heat energy by the heat exchange in at least the cathode air heat exchanger, is supplied to the desulfurization unit, to heat the desulfurization unit by the heat energy of the cathode exhaust gas.