NOVELTY - Fuel cell system (100) comprises: a fuel cell (30) configured to generate electric power with a fuel gas and an oxygen gas fed to discharge an exhaust gas including carbon dioxide; a carbon monoxide extraction part configured to reduce carbon dioxide in the exhaust gas fed to carbon monoxide, where the extraction part includes a processing container fed with the exhaust gas and carbon dioxide adsorbing member formed an oxide with oxygen deficiency; and a carbon monoxide recycling part configured to feed the extracted carbon monoxide to the fuel cell as a part of the fuel gas.    USE - The fuel cell system is used to generate electric power (claimed).    ADVANTAGE - The carbon dioxide discharged from the fuel cell is reduced to carbon monoxide with a simple configuration without inputting external energy as in the conventional electrolysis process, and high energy balance is achieved in a fuel cell system by reusing the carbon monoxide as part of fuel. The carbon dioxide emitted as a result of electric power generation may be reduced by generating electric power using the waste heat of the fuel cell and by culturing algae to serve as a raw material of biofuel e.g. bioethanol using carbon monoxide that has not been reused, which contributes to prevention of global warming.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method of controlling a fuel cell system, comprising: adsorbing a carbon dioxide molecule in an exhaust gas by a carbon dioxide adsorbing member at a first temperature, where the exhaust gas is discharged from a fuel cell as a result of generating electric power in the fuel cell; desorbing a carbon monoxide molecule including a first oxygen atom from the carbon dioxide adsorbing member by increasing a temperature of the carbon dioxide adsorbing member from the first temperature to a second temperature higher than the first temperature; desorbing a second oxygen atom remaining in the carbon dioxide adsorbing member by increasing the temperature of the carbon dioxide adsorbing member from the second temperature to a third temperature higher than the second temperature; and cooling the carbon dioxide adsorbing member to the first temperature, where adsorbing the carbon dioxide molecule, desorbing the carbon monoxide molecule, oxygen atom and cooling the carbon dioxide adsorbing member are sequentially performed during reduction of carbon dioxide in the exhaust gas to carbon monoxide.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of a fuel cell system.    Fuel cell (30)    Water separator (32)    Fuel line (34a)    Air line (35a)    Fuel cell system (100)