A fuel cell device and a method of operating the same, the fuel cell device being provided with a container; a cell stack stored inside the container, the cell stack comprising a plurality of fuel cells operable to generate electric power; 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, operable to measure a measured temperature of the oxygen-containing gas supplied to the fuel cells; and a control unit operable to control the gas supply unit and the power conditioning unit such an amount of oxygen-containing gas supplied from the gas supply unit is increased if the measured temperature is higher than a predetermined temperature.