A fuel cell system (10A to 10D) comprising: a partial oxidation reformer that reforms a raw fuel into a fuel gas by subjecting the raw fuel to partial oxidation reforming; a fuel cell that generates electric power by an electrochemical reaction of the fuel gas produced in the partial oxidation reformer and an oxygen-containing gas; a CO sensor configured to detect a CO concentration in the fuel gas produced in the partial oxidation reformer and introduced into the fuel cell; a mixing ratio adjusting unit configured to adjust a mixing ratio of the raw fuel and air supplied to the partial oxidation reformer; a target CO concentration setting unit configured to set a target CO concentration in the fuel gas introduced into the fuel cell; a mixing control unit configured to control operation of the mixing ratio adjusting unit such that the CO concentration detected by the CO sensor becomes the target CO concentration; and a reformer temperature detecting unit configured to detect a temperature of the partial oxidation reformer; characterized by a fuel cell temperature detecting unit configured to detect a temperature of the fuel cell; a limit CO concentration calculating unit configured to calculate, based on the temperature detected by the fuel cell temperature detecting unit, a limit CO concentration in the fuel gas at which carbon deposition to the fuel cell begins, by using a carbon deposition map defining a relation between the temperature of the fuel cell and the carbon deposition at the fuel cell; and a target mixing ratio calculating unit configured to calculate, from the temperature detected by the reformer temperature detecting unit and the target CO concentration, a target mixing ratio of the raw fuel and air supplied to the partial oxidation reformer, by using a reforming map defining a relation among the temperature of the partial oxidation reformer, the CO concentration in the fuel gas produced in the partial oxidation reformer and introduced into the fuel cell, and the mixing ratio, wherein the target CO concentration setting unit is configured to set the target CO concentration at a concentration lower than the limit CO concentration, wherein the mixing control unit is configured to control the operation of the mixing ratio adjusting unit such that the mixing ratio becomes the target mixing ratio. a partial oxidation reformer that reforms a raw fuel into a fuel gas by subjecting the raw fuel to partial oxidation reforming; a fuel cell that generates electric power by an electrochemical reaction of the fuel gas produced in the partial oxidation reformer and an oxygen-containing gas; a CO sensor configured to detect a CO concentration in the fuel gas produced in the partial oxidation reformer and introduced into the fuel cell; a mixing ratio adjusting unit configured to adjust a mixing ratio of the raw fuel and air supplied to the partial oxidation reformer; a target CO concentration setting unit configured to set a target CO concentration in the fuel gas introduced into the fuel cell; a mixing control unit configured to control operation of the mixing ratio adjusting unit such that the CO concentration detected by the CO sensor becomes the target CO concentration; and a reformer temperature detecting unit configured to detect a temperature of the partial oxidation reformer; characterized by a fuel cell temperature detecting unit configured to detect a temperature of the fuel cell; a limit CO concentration calculating unit configured to calculate, based on the temperature detected by the fuel cell temperature detecting unit, a limit CO concentration in the fuel gas at which carbon deposition to the fuel cell begins, by using a carbon deposition map defining a relation between the temperature of the fuel cell and the carbon deposition at the fuel cell; and a target mixing ratio calculating unit configured to calculate, from the temperature detected by the reformer temperature detecting unit and the target CO concentration, a target mixing ratio of the raw fuel and air supplied to the partial oxidation reformer, by using a reforming map defining a relation among the temperature of the partial oxidation reformer, the CO concentration in the fuel gas produced in the partial oxidation reformer and introduced into the fuel cell, and the mixing ratio, wherein the target CO concentration setting unit is configured to set the target CO concentration at a concentration lower than the limit CO concentration, wherein the mixing control unit is configured to control the operation of the mixing ratio adjusting unit such that the mixing ratio becomes the target mixing ratio.