NOVELTY - A fuel cell system comprises hydrogen production apparatus (2) which is provided with modification part for modifying raw fuel for hydrogen production and generating hydrogen rich reformed gas and carbon monoxide removal part with selective oxidation device for selectively oxidizing carbon monoxide remained in reformed gas to change into carbon dioxide, and fuel cell stack (4) which introduces reformed gas manufactured by hydrogen production apparatus into anode, introduces air into cathode and generates electromotive force.    USE - Fuel cell system.    ADVANTAGE - The compactization and cost reduction of fuel cell system are attained. The required electric power generation amount is remarkably maintained without impairing gross efficiency of fuel cell system.    DETAILED DESCRIPTION - A fuel cell system comprises hydrogen production apparatus (2) which is provided with modification part for modifying raw fuel for hydrogen production and generating hydrogen rich reformed gas and carbon monoxide removal part with selective oxidation device for selectively oxidizing carbon monoxide remained in reformed gas to change into carbon dioxide, fuel cell stack (4) equipped with cathode which introduces reformed gas manufactured by hydrogen production apparatus into anode, introduces air into cathode and generates electromotive force, air supply apparatus which supplies air, branch pipe which branches air supply pipe and air pump, air supply pipe which connects air supply apparatus to one set of air pump, and control part which feedback-controls parameter that adjusts air fuel ratio and each temperature so that each temperature of modification part and selective oxidation device might be maintained within each setting. The modification part has burner for burning fuel for combustion. The air fuel ratio of fuel for combustion and air supplied to burner satisfy required electric power generation amount of fuel cell stack.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the fuel cell system. (Drawing includes non-English language text).    Raw fuel pump for hydrogen production (1)    Hydrogen production apparatus (2)    Fuel feed pump for burner combustion (3)    Polymer electrolyte fuel cell stack (4)    Cooling agent pump (5)