In a fuel reforming system (32) of a fuel cell system (1), which includes a reformer (7) generating hydrogen-containing gas to be an electromotive fuel and a combustor (6) generating combustion gas, the combustor (6) has two combustion parts including a main combustion part (20) and 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) which supply the fuel to the respective combustion parts. In start-up of the fuel cell system (1), predetermined amounts of fuel and air for generating heat quantity required in the fuel pre-vaporization part (19) are subjected to lean combustion in the sub-combustion part (18), the fuel to be combusted in the main combustion part (20) is pre-vaporized in the fuel pre-vaporization part (19) by use of the combustion gas from the sub-combustion part (18), and the pre-vaporized fuel obtained by the pre-vaporization and air are subjected to premixed lean combustion in the main combustion part (20).