To provide a fuel cell system which efficiently utilizes exhaust heat exhausted from a high temperature cell stack and reformer for desulfurization of raw fuel, and exhibits high efficiency of heat utilization.

The fuel cell system 1 includes: a desulfurizer 10 which desulfurizes the raw fuel to make desulfurized raw fuel; a raw fuel supply means for supplying the raw fuel to the desulfurizer 10; the reformer 3 for reforming the desulfurized raw fuel and producing reformed gas; a desulfurized raw fuel supply means for supplying the desulfurized raw fuel from the desulfurizer 10 to the reformer 3; the fuel cell stack 4 formed by combining a plurality of cells for a solid oxide fuel cell; and a reformed gas supplying means for supplying the reformed gas from the reformer 3 to the fuel cell stack 4. The fuel cell system 1 further includes: an inner wall 5 on which a high temperature chamber 6, in which the reformer 3 and/or fuel cell stack 1 are arranged, is formed inside; and an outer wall 7 which covers an outer circumference of the inner wall 5 and forms a heat insulation chamber 8 with the inner wall 5. The desulfurizer 10 and a heat insulation structure are arranged in the heat insulation chamber 8.

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