A fuel reforming system has a reformer (4) for reforming a fuel to produce a reformate gas; a shift converter (5) for reacting carbon monoxide (CO) contained in a reformate gas with water to produce hydrogen (H.sub.2); a CO oxidizer (6) which removes CO discharged from the shift converter; and a startup combustor (11) for supplying combustion gas to the reformer (4), a shift converter (5) and CO oxidizer (6) to warm up them. An NOx trap (16) is disposed downstream of the startup combustor so as to adsorb nitrogen oxides (NOx) in combustion gas. A fuel reforming system further has a controller for controlling warm-up operations. When the warm-up operation for the reformer, the shift converter and the CO oxidizer is completed, the reformate reactions are commenced in the reformer (4). The NOx trapped by the NOx trap is decomposed by the reformate gas which contains CO gas and H.sub.2 gas.