A fuel cell system (100, 200) comprising: a reforming section that generates fuel from water for reforming and a raw reforming material; a fuel feeding section (11a5) that feeds the raw reforming material to the reforming section; a fuel cell that generates electricity using the fuel generated by the reforming section, and an oxidant gas; a combusting section that combusts an anode off gas containing a combustible component which is discharged from the fuel cell, with the oxidant gas; a catalytic combustor into which a combustion gas is introduced and that combusts the combustible component contained in the combustion gas using a catalytic reaction, the combustion gas containing the combustible component which is discharged from the combusting section and is not combusted; a temperature detecting section that detects a temperature of the catalytic combustor; and an oxygen proportion changing section that changes an oxygen proportion to increase a flow rate of the combustible component which is introduced into the catalytic combustor from the combusting section and is not combusted, compared to the flow rate of the combustible component before the oxygen proportion is changed, in which the oxygen proportion is obtained by dividing a flow rate of oxygen introduced into the combusting section by an ideal oxygen flow rate being necessary for causing the combustible component introduced into the combusting section to be completely combusted, characterized by a determining section that determines whether or not the catalytic reaction occurring in the catalytic combustor is degraded based on the temperature of the catalytic combustor, which is detected by the temperature detecting section, when the oxygen proportion changing section decreases the oxygen proportion so as to become a determination oxygen proportion lower than an incomplete combustion start oxygen proportion which causes the combustible component to be incompletely combusted in the combusting section. a reforming section that generates fuel from water for reforming and a raw reforming material; a fuel feeding section (11a5) that feeds the raw reforming material to the reforming section; a fuel cell that generates electricity using the fuel generated by the reforming section, and an oxidant gas; a combusting section that combusts an anode off gas containing a combustible component which is discharged from the fuel cell, with the oxidant gas; a catalytic combustor into which a combustion gas is introduced and that combusts the combustible component contained in the combustion gas using a catalytic reaction, the combustion gas containing the combustible component which is discharged from the combusting section and is not combusted; a temperature detecting section that detects a temperature of the catalytic combustor; and an oxygen proportion changing section that changes an oxygen proportion to increase a flow rate of the combustible component which is introduced into the catalytic combustor from the combusting section and is not combusted, compared to the flow rate of the combustible component before the oxygen proportion is changed, in which the oxygen proportion is obtained by dividing a flow rate of oxygen introduced into the combusting section by an ideal oxygen flow rate being necessary for causing the combustible component introduced into the combusting section to be completely combusted, characterized by a determining section that determines whether or not the catalytic reaction occurring in the catalytic combustor is degraded based on the temperature of the catalytic combustor, which is detected by the temperature detecting section, when the oxygen proportion changing section decreases the oxygen proportion so as to become a determination oxygen proportion lower than an incomplete combustion start oxygen proportion which causes the combustible component to be incompletely combusted in the combusting section.