NOVELTY - An impurity removal apparatus has 1st catalyst for oxidation reaction to remove oxygen contained in fuel, 2nd catalyst for converting aromatic compound contained in fuel into saturated hydrocarbon by hydrogenation reaction, 3rd catalyst for converting sulfur compound contained in fuel into hydrogen sulfide by hydrogenation reaction, and 4th catalyst for removing hydrogen sulfide produced.    USE - Impurity removal apparatus in fuel cell system which uses fuel which contains oxygen, aromatic compound, and sulfur compound as impurities, as raw fuel (claimed).    ADVANTAGE - Effectivity is stable over a long period of time.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) fuel reforming system comprises 1st impurity removal device, modifier connected to the 1st impurity removal device for converting fuel into reformed gas which contains hydrogen, carbon monoxide (CO)-modifying device connected to the modifier for removing a part of carbon monoxide contained in the reformed gas, CO removal device connected with CO-modifying device for removing remaining carbon monoxide, and recycle line which connects CO-modifying device and 1st impurity removal device for introducing a part of reformed gas from the CO-modifying device into the 1st impurity removal device;    (2) operating method of fuel reforming system which involves removing oxygen, aromatic compound, and sulfur compound contained in raw fuel in the 2nd impurity removal device when the temperature of 1st impurity removal device is less than or equal to 200 degrees C, and removing oxygen, aromatic compound, and sulfur compound contained in the raw fuel in the 1st impurity removal device, When the temperature of 1st impurity removal device is 200-350 degrees C; and    (3) fuel cell system comprising fuel reforming system, and fuel cell generated using the fuel modified with the fuel reforming system.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional diagram of impurity removal apparatus. (Drawing includes non-English language text).    1st catalyst layer (1)    2nd catalyst layer (2)    3rd catalyst layer (3)    4th catalyst layer (4)    Fuel inlet (5)