A fuel cell system, comprising: a fuel cell including an anode and a cathode; a cathode gas passage supplying a cathode gas to the cathode of the fuel cell; a reformer reforming a source gas to generate an anode gas; a source gas passage supplying the source gas from a gas source to the reformer; a desulfurization unit arranged at the source gas passage and desulfurizing the source gas; an anode gas passage supplying the anode gas generated by the reformer to the anode of the fuel cell; a power generation module formed by the fuel cell and the reformer covered by an insulated wall; a hot-water passage through which hot water heated by a power generating operation of the fuel cell system flows; and a hot-water chamber connected to the hot-water passage and storing the hot water supplied from the hot-water passage, the desulfurization unit comprising: a first desulfurizer arranged in a temperature environment ranging from 50 °C to 200 °C and accommodating a normal-temperature desulfurizing agent including a porous material serving as a base material, the normal-temperature desulfurizing agent exerting a desulfurization effect in a normal temperature range that ranges from 0 °C to 50 °C exclusive of 50 °C; and a second desulfurizer arranged in a temperature environment that is lower than 50 °C and accommodating the normal-temperature desulfurizing agent, the desulfurization unit adsorbing a sulfur compound included in the source gas having a low dew point and the source gas having a high dew point as compared to the source gas having the low dew point, the second desulfurizer being positioned in series with the first desulfurizer and at an upstream side or a downstream side of the first desulfurizer in a flow direction of the source gas, the first desulfurizer receiving heat from the reformer or the power generation module, or heat from a combustion gas exhaust passage receiving a combustion gas emitted from the reformer or the power generation module, or heat from at least one of the hot-water passage and the hot-water chamber to reach the temperature ranging from 50 °C to 200 °C. a fuel cell including an anode and a cathode; a cathode gas passage supplying a cathode gas to the cathode of the fuel cell; a reformer reforming a source gas to generate an anode gas; a source gas passage supplying the source gas from a gas source to the reformer; a desulfurization unit arranged at the source gas passage and desulfurizing the source gas; an anode gas passage supplying the anode gas generated by the reformer to the anode of the fuel cell; a power generation module formed by the fuel cell and the reformer covered by an insulated wall; a hot-water passage through which hot water heated by a power generating operation of the fuel cell system flows; and a hot-water chamber connected to the hot-water passage and storing the hot water supplied from the hot-water passage, the desulfurization unit comprising: a first desulfurizer arranged in a temperature environment ranging from 50 °C to 200 °C and accommodating a normal-temperature desulfurizing agent including a porous material serving as a base material, the normal-temperature desulfurizing agent exerting a desulfurization effect in a normal temperature range that ranges from 0 °C to 50 °C exclusive of 50 °C; and a second desulfurizer arranged in a temperature environment that is lower than 50 °C and accommodating the normal-temperature desulfurizing agent, the desulfurization unit adsorbing a sulfur compound included in the source gas having a low dew point and the source gas having a high dew point as compared to the source gas having the low dew point, the second desulfurizer being positioned in series with the first desulfurizer and at an upstream side or a downstream side of the first desulfurizer in a flow direction of the source gas, the first desulfurizer receiving heat from the reformer or the power generation module, or heat from a combustion gas exhaust passage receiving a combustion gas emitted from the reformer or the power generation module, or heat from at least one of the hot-water passage and the hot-water chamber to reach the temperature ranging from 50 °C to 200 °C. a first desulfurizer arranged in a temperature environment ranging from 50 °C to 200 °C and accommodating a normal-temperature desulfurizing agent including a porous material serving as a base material, the normal-temperature desulfurizing agent exerting a desulfurization effect in a normal temperature range that ranges from 0 °C to 50 °C exclusive of 50 °C; and a second desulfurizer arranged in a temperature environment that is lower than 50 °C and accommodating the normal-temperature desulfurizing agent, the desulfurization unit adsorbing a sulfur compound included in the source gas having a low dew point and the source gas having a high dew point as compared to the source gas having the low dew point, the second desulfurizer being positioned in series with the first desulfurizer and at an upstream side or a downstream side of the first desulfurizer in a flow direction of the source gas, the first desulfurizer receiving heat from the reformer or the power generation module, or heat from a combustion gas exhaust passage receiving a combustion gas emitted from the reformer or the power generation module, or heat from at least one of the hot-water passage and the hot-water chamber to reach the temperature ranging from 50 °C to 200 °C.