A fuel cell system comprising: a fuel cell generating an electric power by an oxidant gas and a reformed gas including a hydrogen; an evaporation portion generating a water vapor from a water for reforming; a reforming portion generating the reformed gas by a source material and the water vapor and supplying the reformed gas to the fuel cell; a desulfurizer (11a6) removing a sulfur component contained in the source material by the hydrogen and supplying the source material in which the sulfur component is removed to the reforming portion; a recycle gas pipe connecting between a reformed gas supply pipe which supplies the reformed gas to the fuel cell from the reforming portion and a source material supply pipe which supplies the source material to the desulfurizer (11a6), the recycle gas pipe returning a portion of the reformed gas to the desulfurizer (11a6) as a recycle fuel; a drain pipe including a drain water inlet port at a first end and a drain water outlet port at a second end, the drain water inlet port introducing a drain water generated at the recycle gas pipe, the drain water outlet port sending out the drain water; a water tank storing therein water and connected to the drain water outlet port at a lower side than the drain water inlet port, the water tank configured in a manner that a water level of the water tank is adjustable to be equal to or greater than a predetermined water level (Ls); and a control unit, characterized by a water level detection device detecting a water level of the water tank; and an interruption device (11a1) provided at the source material supply pipe and configured to interrupt a supply of the source material, wherein the water tank includes a reform water inlet port introducing therein the drain water as the water for reforming, the predetermined water level (Ls) serves as a water level of the water tank obtained in a case where the reformed gas at a predetermined pressure (Ps) related to a maximum pressure is applied to a boundary surface (Sk) between the reformed gas and the water for reforming within the drain pipe and the boundary surface (Sk) is positioned at a same height as a sealing water limit surface (Sh) serving as a limit surface for the reformed gas not flowing into the water tank, and the control unit includes an interruption control portion interrupting the supply of the source material by the interruption device (11a1) in a case where the water level of the water tank detected by the water level detection device is smaller than the predetermined water level (Ls) in a state where the reformed gas is introduced to the drain pipe from the recycle gas pipe. a fuel cell generating an electric power by an oxidant gas and a reformed gas including a hydrogen; an evaporation portion generating a water vapor from a water for reforming; a reforming portion generating the reformed gas by a source material and the water vapor and supplying the reformed gas to the fuel cell; a desulfurizer (11a6) removing a sulfur component contained in the source material by the hydrogen and supplying the source material in which the sulfur component is removed to the reforming portion; a recycle gas pipe connecting between a reformed gas supply pipe which supplies the reformed gas to the fuel cell from the reforming portion and a source material supply pipe which supplies the source material to the desulfurizer (11a6), the recycle gas pipe returning a portion of the reformed gas to the desulfurizer (11a6) as a recycle fuel; a drain pipe including a drain water inlet port at a first end and a drain water outlet port at a second end, the drain water inlet port introducing a drain water generated at the recycle gas pipe, the drain water outlet port sending out the drain water; a water tank storing therein water and connected to the drain water outlet port at a lower side than the drain water inlet port, the water tank configured in a manner that a water level of the water tank is adjustable to be equal to or greater than a predetermined water level (Ls); and a control unit, characterized by a water level detection device detecting a water level of the water tank; and an interruption device (11a1) provided at the source material supply pipe and configured to interrupt a supply of the source material, wherein the water tank includes a reform water inlet port introducing therein the drain water as the water for reforming, the predetermined water level (Ls) serves as a water level of the water tank obtained in a case where the reformed gas at a predetermined pressure (Ps) related to a maximum pressure is applied to a boundary surface (Sk) between the reformed gas and the water for reforming within the drain pipe and the boundary surface (Sk) is positioned at a same height as a sealing water limit surface (Sh) serving as a limit surface for the reformed gas not flowing into the water tank, and the control unit includes an interruption control portion interrupting the supply of the source material by the interruption device (11a1) in a case where the water level of the water tank detected by the water level detection device is smaller than the predetermined water level (Ls) in a state where the reformed gas is introduced to the drain pipe from the recycle gas pipe.