To provide a fuel cell system with a simple constitution by which combustion failure of a combustor and leakage of combustible gas in a cabinet can be detected.

This includes a fuel treater 1 to form fuel gas containing hydrogen by reforming reaction of a raw material gas, a fuel cell 4 to carry out power generation by using the fuel gas, the combustor 2 to heat the fuel treater 1, a combustion air supplier 8 to supply combustion air to the combustor 2, a combustion exhaust gas passage 5 in which the combustion exhaust gas discharged from the combustor 2 flows, the cabinet 9 to cover the fuel treater 1, the fuel cell 4, the combustor 2, the combustion air supplier 8, and the combustion exhaust gas passage 5, a first ventilator 7 in order to discharge gas in the cabinet 9 to the outside of the cabinet 9, and a combustible gas detector 6 installed in the cabinet 9. The exit of the combustion exhaust gas passage 5 is opened in the cabinet 9, and the combustible gas detector 6 is installed on the flow of the combustion exhaust gas which is discharged from the exit until the gas reaches the ventilator 7.