A fuel cell system comprising: a reformer (14, 114) configured to generate a reformed gas from a raw material gas, water, and air that are supplied to the reformer (14, 114), the reformed gas serving as a fuel gas; a fuel cell configured to generate electric power through a power-generating reaction by utilizing the fuel gas and air; a combustor configured to combust an anode off gas, the anode off gas being the fuel gas that has not been utilized in the fuel cell; a casing part housing at least the reformer (14, 114), the fuel cell, and the combustor, which are covered with a heat insulating material; and a desulfurizer (20, 120) configured to remove a sulfur component from the raw material gas supplied thereto by hydrodesulfurization, wherein the anode off gas is supplied to the combustor and the desulfurizer (20, 120) in a distributed manner, and the desulfurizer (20, 120) performs the hydrodesulfurization of the raw material gas by utilizing the supplied anode off gas as a hydrogen source and utilizing an exhaust gas discharged from the casing part as a heat source, the exhaust gas containing at least combustion heat from the combustor, characterized in that the fuel cell system further comprises: an air supplying unit configured to adjust a flow rate of the air supplied to the reformer (14, 114); a temperature detector configured to detect a temperature of a desulfurization catalyst packed in the desulfurizer (20, 120); a controller configured to control the flow rate of the air supplied by the air supplying unit based on a result of detection by the temperature detector; and a distributor configured to distribute the anode off gas to the combustor and the desulfurizer (20, 120) while adjusting an amount of the anode off gas supplied to the combustor and an amount of the anode off gas supplied to the desulfurizer (20, 120), wherein the controller controls the flow rate of the air supplied by the air supplying unit based on the result of the detection by the temperature detector, and controls a distribution ratio of the anode off gas distributed by the distributor. a reformer (14, 114) configured to generate a reformed gas from a raw material gas, water, and air that are supplied to the reformer (14, 114), the reformed gas serving as a fuel gas; a fuel cell configured to generate electric power through a power-generating reaction by utilizing the fuel gas and air; a combustor configured to combust an anode off gas, the anode off gas being the fuel gas that has not been utilized in the fuel cell; a casing part housing at least the reformer (14, 114), the fuel cell, and the combustor, which are covered with a heat insulating material; and a desulfurizer (20, 120) configured to remove a sulfur component from the raw material gas supplied thereto by hydrodesulfurization, wherein the anode off gas is supplied to the combustor and the desulfurizer (20, 120) in a distributed manner, and the desulfurizer (20, 120) performs the hydrodesulfurization of the raw material gas by utilizing the supplied anode off gas as a hydrogen source and utilizing an exhaust gas discharged from the casing part as a heat source, the exhaust gas containing at least combustion heat from the combustor, an air supplying unit configured to adjust a flow rate of the air supplied to the reformer (14, 114); a temperature detector configured to detect a temperature of a desulfurization catalyst packed in the desulfurizer (20, 120); a controller configured to control the flow rate of the air supplied by the air supplying unit based on a result of detection by the temperature detector; and a distributor configured to distribute the anode off gas to the combustor and the desulfurizer (20, 120) while adjusting an amount of the anode off gas supplied to the combustor and an amount of the anode off gas supplied to the desulfurizer (20, 120), wherein the controller controls the flow rate of the air supplied by the air supplying unit based on the result of the detection by the temperature detector, and controls a distribution ratio of the anode off gas distributed by the distributor.