NOVELTY - Fuel cell system comprises a fuel processor comprising a reformer and a shift reactor; fuel cell stack; carbon monoxide adsorbing device comprising a carbon monoxide adsorbent; first supplying unit to selectively supply carbon monoxide rich gas from the shift reactor to the carbon monoxide adsorbing device; second supplying unit to selectively supply carbon monoxide lean gas obtained from the carbon monoxide adsorbing device to the fuel cell stack; and third supplying unit to selectively supply carbon monoxide lean gas obtained from the shift reactor directly to the fuel cell stack.    USE - As fuel cell system.    ADVANTAGE - A start-up time of the fuel cell system is reduced without poisoning catalysts of electrodes of the fuel cell. An overall volume of the fuel cell system can be reduced since the carbon monoxide adsorbing device is only operating during the start-up, and thus the fuel cell system can be economically manufactured and operated. The carbon monoxide adsorbent can be regenerated, thus increasing economic efficiency of the fuel cell system.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for operating the fuel cell system, comprising:    (a) a start-up operation comprising supplying the carbon monoxide rich gas from the shift reactor to the carbon monoxide adsorbing device via the first supplying unit, supplying the carbon monoxide lean gas obtained from the carbon monoxide adsorbing device to the fuel cell stack via the second supplying unit, and blocking the third supplying unit; and    (b) a steady-state operation comprising blocking the first and second supplying units when the outlet temperature of the shift reactor is 150degreesC, and supplying the carbon monoxide lean gas obtained from the shift reactor to the fuel cell stack via the third supplying unit.    The method also comprises a regeneration operation comprising regenerating the carbon monoxide adsorbent (for 10-120 minutes) by supplying a desorption gas to the carbon monoxide adsorbing device via a fourth supplying unit and discharging the desorbed carbon monoxide gas from the carbon monoxide adsorbent in the carbon monoxide adsorbing device via a first discharge unit.    DESCRIPTION OF DRAWING(S) - The figure shows a flow diagram of an operation of a fuel cell system.