A fuel cell system comprising: a fuel cell generating an electric power by a fuel and an oxidant gas; a reforming portion generating the fuel by a source material and water to supply the fuel to the fuel cell; a first combusting portion receiving a combustible gas including an unused fuel from the fuel cell to burn the combustible gas by an oxidant gas and sending out a combustion gas; a second combusting portion receiving an unused combustible gas discharged from the first combusting portion to burn the unused combustible gas and sending out a combustion gas; and a determination portion (60,108-112) performing a determination process in which the determination portion (60,108-112) determines that the second combusting portion is in a normal state in a case where a temperature change (ΔT1) of the second combusting portion is equal to or greater than a predetermined value (ΔT1-a) in a state where the first combusting portion is inhibited from being burnt while being provided with the combustible gas and the oxidant gas, the determination portion (60,108-112) determining that the second combusting portion is in an error state in a case where the temperature change (ΔT1) of the second combusting portion is smaller than the predetermined value (ΔT1-a) in a state where the first combusting portion is inhibited from being burnt while being provided with the combustible gas and the oxidant gas, wherein during a stop operation of the fuel cell system the determination portion (60,108-112) performs the determination process in a state where the source material is supplied to the first combusting portion at a first predetermined flow rate (αNLM) that is smaller than a flow rate for each of a warm-up operation and a stable operation of the fuel cell system, and the oxidant gas is supplied to the first combusting portion at a second predetermined flow rate (γNLM) that is greater than a flow rate for each of the warm-up operation and the stable operation. a fuel cell generating an electric power by a fuel and an oxidant gas; a reforming portion generating the fuel by a source material and water to supply the fuel to the fuel cell; a first combusting portion receiving a combustible gas including an unused fuel from the fuel cell to burn the combustible gas by an oxidant gas and sending out a combustion gas; a second combusting portion receiving an unused combustible gas discharged from the first combusting portion to burn the unused combustible gas and sending out a combustion gas; and a determination portion (60,108-112) performing a determination process in which the determination portion (60,108-112) determines that the second combusting portion is in a normal state in a case where a temperature change (ΔT1) of the second combusting portion is equal to or greater than a predetermined value (ΔT1-a) in a state where the first combusting portion is inhibited from being burnt while being provided with the combustible gas and the oxidant gas, the determination portion (60,108-112) determining that the second combusting portion is in an error state in a case where the temperature change (ΔT1) of the second combusting portion is smaller than the predetermined value (ΔT1-a) in a state where the first combusting portion is inhibited from being burnt while being provided with the combustible gas and the oxidant gas,