A fuel cell system, comprising: a fuel cell that generates electricity using a fuel and an oxidant gas; a reforming unit that generates the fuel from a reforming raw material and reforming water, and supplies the fuel to the fuel cell; a combustion unit to which a combustible gas including the fuel that is not used is introduced from the fuel cell to combust the combustible gas with the oxidant gas, and from which a combustion gas is output; a raw material supply device (11a6) that supplies the reforming raw material; an oxidant gas supply device (11c1) that supplies the oxidant gas; a flow rate detecting sensor (11a3) that detects a flow rate of the reforming raw material that is supplied to the reforming unit; a first temperature sensor that detects a temperature of the reforming unit; a second temperature sensor that detects a temperature of the combustion unit; and a control device which determines that a gas component of the reforming raw material varies in a case where the flow rate of the reforming raw material which is detected by the flow rate detecting sensor (11a3) is in a first predetermined range, and the temperature of the reforming unit which is detected by the first temperature sensor, and the temperature of the combustion unit which is detected by the second temperature sensor are equal to or lower than a lower limit of a second predetermined range and equal to or lower than a lower limit of a third predetermined range, respectively, and which changes operation conditions of the fuel cell system, wherein the control device changes the operation conditions of the fuel cell system so as to decrease threshold values which are set to define the predetermined ranges of the first temperature sensor and the second temperature sensor. a fuel cell that generates electricity using a fuel and an oxidant gas; a reforming unit that generates the fuel from a reforming raw material and reforming water, and supplies the fuel to the fuel cell; a combustion unit to which a combustible gas including the fuel that is not used is introduced from the fuel cell to combust the combustible gas with the oxidant gas, and from which a combustion gas is output; a raw material supply device (11a6) that supplies the reforming raw material; an oxidant gas supply device (11c1) that supplies the oxidant gas; a flow rate detecting sensor (11a3) that detects a flow rate of the reforming raw material that is supplied to the reforming unit; a first temperature sensor that detects a temperature of the reforming unit; a second temperature sensor that detects a temperature of the combustion unit; and a control device which determines that a gas component of the reforming raw material varies in a case where the flow rate of the reforming raw material which is detected by the flow rate detecting sensor (11a3) is in a first predetermined range, and the temperature of the reforming unit which is detected by the first temperature sensor, and the temperature of the combustion unit which is detected by the second temperature sensor are equal to or lower than a lower limit of a second predetermined range and equal to or lower than a lower limit of a third predetermined range, respectively, and which changes operation conditions of the fuel cell system, wherein the control device changes the operation conditions of the fuel cell system so as to decrease threshold values which are set to define the predetermined ranges of the first temperature sensor and the second temperature sensor.