NOVELTY - A reformer (2) has comprises a modification portion (31) for producing reformed gas containing carbon monoxide using modification fuel containing hydrogen, carbon monoxide oxidation removal portion (37) having catalyst (37e), oxygen supply portion (72), and control apparatus for performing regeneration of catalyst of removal portion. Reformed gas containing hydrogen is remained in carbon monoxide oxidation removal portion. Oxygen component is supply to the removal portion, and catalyst of the removal portion is maintained to catalyst regeneration temperature region.    USE - Reformer is used for producing reformed gas.    ADVANTAGE - The reformer has improved durability. The catalyst is efficiently regenerated.    DETAILED DESCRIPTION - A reformer has comprises a modification portion for producing reformed gas containing carbon monoxide using modification fuel containing hydrogen, carbon monoxide oxidation removal portion having catalyst, oxygen supply portion for supplying oxygen component to the removal portion, and control apparatus for performing regeneration of catalyst of removal portion. The catalyst reduces carbon monoxide from reformed gas by oxidizing carbon monoxide in oxygen-containing atmosphere at predetermined temperature region (I). Reformed gas containing hydrogen is remained in carbon monoxide oxidation removal portion. Oxygen component is supply to the removal portion, and catalyst of the removal portion is maintained to catalyst regeneration temperature region and regenerated.    DESCRIPTION OF DRAWING(S) - The drawing shows the systematic diagram of reformer. (Drawing includes non-English language text)    Reformer (2)    Outlet valve (25e,25f)    Heating portion (30)    Modification portion (31)    Carbon monoxide oxidation removal portion (37)    Catalyst (37e)    Carbon monoxide oxidation removal portion temperature detector (38)    Heat insulation portion (39)    Carbon monoxide shift portion temperature detector (55)    Oxygen supply portion (72)    Reformed gas discharge path (100)