The invention presents a fuel reforming technique for a mobile fuel cell system capable of obtaining a reformed gas composition usable in fuel cell 200 even if vapor temperature supplied from an evaporator 102 into a fuel reformer 107 varies significantly.This system comprises means 601, 602 for detecting the flow rate of fuel vapor and oxygen to be supplied into the fuel reformer 107, and means 600 for detecting at least temperature of fuel vapor to be supplied into the fuel reformer, temperature of oxygen, and temperature of mixed gas of fuel vapor and oxygen, in which the ratio of the flow rate of fuel vapor and the flow rate of oxygen is corrected on the basis of the signal value of the temperature detecting means, and oxygen is supplied depending on the corrected ratio.