NOVELTY - A fuel cell system (100) has hydrogen production apparatus (1) comprising a desulfurized portion which desulfurizes raw fuel containing nitrogen (0.01 vol.% or more), a modification portion (6) which generates reformed gas containing hydrogen by modifying desulfurized raw fuel by a modification catalyst and selective oxidation reaction portion, and a recycle line (RL1-RL3). The modification catalyst contains alumina, and support obtained by supporting alumina-rare-earth-element oxide (in %mass) (2-20), alkaline-earth element oxide (0.1-10) and nickel (1-30) with respect to alumina content.    USE - Fuel cell system.    ADVANTAGE - The fuel cell system efficiently recycles reformed gas and/or off gas. The generation of ammonia in modification portion of fuel cell system is suppressed.    DETAILED DESCRIPTION - A fuel cell system has hydrogen production apparatus comprising a desulfurized portion which desulfurizes raw fuel containing nitrogen (0.01 vol.% or more), a modification portion which generates reformed gas containing hydrogen by modifying desulfurized raw fuel by a modification catalyst and selective oxidation reaction portion which carries out selective oxidation of carbon monoxide in reformed gas, fuel cell stack (20) which generates electric power using reformed gas, and a recycle line which circulates gas containing hydrogen in downstream rather than the modification portion to desulfurized portion. The modification catalyst contains alumina, and support obtained by supporting alumina-rare-earth-element oxide (in %mass) (2-20), alkaline-earth element oxide (0.1-10) and nickel (1-30) with respect to alumina content. The platinum-group element is further supported. The content of platinum-group element is 0.01-3 %mass per alumina content, and the platinum-group element is chosen from palladium and platinum.    DESCRIPTION OF DRAWING(S) - The drawing shows the block diagram of fuel cell system. (Drawing includes non-English language text)    Hydrogen production apparatus (1)    Modification portion (6)    Fuel cell stack (20)    Fuel cell system (100)    Recycle lines (RL1-RL3)