A fuel cell device, comprising: a container; a cell stack stored inside the container, the cell stack comprising a plurality of fuel cells operable to generate electric power with fuel gas and oxygen-containing gas; a gas supply unit to supply oxygen-containing gas to the fuel cells; a supplying-power conditioning unit operable to adjust the quantity of generated electric power in the cell stack; a thermal sensor outside the container, configured to measure a measured temperature of the oxygen-containing gas supplied to the fuel cells; and a control unit configured to control the gas supply unit and the supplying-power conditioning unit; characterized in that the control unit is configured to control a relationship between a rate of air utilization (Ua) of the cell stack and the amount of current generated by the cell stack such that: during a rated operation, the rate of air utilization Ua is kept constant in response to the demands from the external load even if the amount of current varies; during a partial load operation the rate of air utilization (Ua) is changed depending on the amount of current of the cell stack in response to the required load; and the control unit is configured to control so as to increase an amount of oxygen-containing gas supplied from the gas supply unit if the temperature of the oxygen-containing gas measured by the thermal sensor is higher than a predetermined temperature. a container; a cell stack stored inside the container, the cell stack comprising a plurality of fuel cells operable to generate electric power with fuel gas and oxygen-containing gas; a gas supply unit to supply oxygen-containing gas to the fuel cells; a supplying-power conditioning unit operable to adjust the quantity of generated electric power in the cell stack; a thermal sensor outside the container, configured to measure a measured temperature of the oxygen-containing gas supplied to the fuel cells; and a control unit configured to control the gas supply unit and the supplying-power conditioning unit; characterized in that the control unit is configured to control a relationship between a rate of air utilization (Ua) of the cell stack and the amount of current generated by the cell stack such that: during a rated operation, the rate of air utilization Ua is kept constant in response to the demands from the external load even if the amount of current varies; during a partial load operation the rate of air utilization (Ua) is changed depending on the amount of current of the cell stack in response to the required load; and the control unit is configured to control so as to increase an amount of oxygen-containing gas supplied from the gas supply unit if the temperature of the oxygen-containing gas measured by the thermal sensor is higher than a predetermined temperature. during a rated operation, the rate of air utilization Ua is kept constant in response to the demands from the external load even if the amount of current varies; during a partial load operation the rate of air utilization (Ua) is changed depending on the amount of current of the cell stack in response to the required load; and the control unit is configured to control so as to increase an amount of oxygen-containing gas supplied from the gas supply unit if the temperature of the oxygen-containing gas measured by the thermal sensor is higher than a predetermined temperature.