NOVELTY - A fuel reforming system of a fuel cell system comprises a fuel reformer to generate hydrogen-containing gas; and a combustor including a main combustion part (20), a sub-combustion part (18), a fuel pre-vaporization part (19) between the two combustion parts, and a main fuel injection valve (16) and a sub-fuel injection valve (15) for supplying liquid fuel to respective combustion parts.    USE - For a fuel cell system (claimed) useful in a vehicle drive power source.    ADVANTAGE - The fuel reforming system of fuel cell system provides improved exhaust performance, start-up performance, stopping performance and transient response. The system performs combustion such that the generation amount of nitrogen oxides in the main combustion part is extremely lowered. Therefore, the emission of nitrogen oxides from the combustor can be drastically reduced, due to reduction in the total amount of nitrogen oxides generated in the sub-combustion part and the main combustion part. In addition, by distributing the high-temperature combustion gas generated in the fuel reforming system, a smooth start-up of the system is obtained.    DETAILED DESCRIPTION - When the fuel system starts, predetermined amounts of the fuel and air are subjected to a lean combustion in the sub-combustion part, for generating heat in the fuel pre-vaporization part. The fuel to be combusted in the main combustion part is pre-vaporized in the fuel pre-vaporization part by using combustion gas generated by the lean combustion in sub-combustion part. The air and the pre-vaporized fuel are then subjected to premixed lean combustion in the main combustion part. The system further comprises a heat exchanger (21) downstream of the main combustion part.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of a combustor of a fuel reforming system.    sub-fuel injection valve (15)    main fuel injection valve (16)    sub-combustion part (18)    fuel pre-vaporization part (19)    main combustion part (20)    heat exchanger (21)    gas temperature sensors. (24 - 26)