NOVELTY - A fuel reformer comprises reformer (1), carbon monoxide (CO) removal apparatus (3,4) packed with catalyst, and heat exchanger (2). Coolant (A) is circulated to heat exchanger and coolant (B) is circulated to CO removal apparatus. Circulation apparatuses (I,II) for controlling temperature of respective coolants (A,B) are provided. Temperature of coolant (A) is set lower than that of coolant (B).    USE - For producing reformed gas containing hydrogen.    ADVANTAGE - By cooling reformed gas before introduction into carbon monoxide removal apparatus, oxidation reaction at the inlet of carbon monoxide removal apparatus is inhibited and increase in catalyst temperature is prevented. By setting the temperature of coolant (B) more than the active temperature of catalyst layer, catalyst can be maintained to active temperature by simple process. By using the common heat exchanger for circulation apparatus, size reduction and cost reduction of system is enabled. By reducing the flow rate of coolant circulated to heat exchanger and by increasing the temperature of reformed gas, reaction at the inlet of carbon monoxide removal apparatus is eliminated and concentration of carbon monoxide is not reduced. By circulating coolant (A) to the fuel cell stack, temperature raise of fuel cell stack is prevented. The side reaction are inhibited. The efficiency of the system is improved. The temperature of the reformed gas circulated to carbon monoxide removal apparatus can be changed to target value by controlling the temperature and flow rate of the coolant.    DETAILED DESCRIPTION - A fuel reformer comprises reformer equipped with catalyst layer for forming reformer gas containing hydrogen, at least one carbon monoxide removal apparatus packed with catalyst layer for removing carbon monoxide from reformed gas, and heat exchanger between reformer and carbon monoxide removal apparatus. Coolant (A) is circulated to heat exchanger and coolant (B) is circulated among heat exchange portion of carbon monoxide removal apparatus. Circulation apparatuses (I,II) for controlling temperature of respective coolants (A,B) are provided. Temperature of coolant (A) is set lower than temperature of coolant (B).    DESCRIPTION OF DRAWING(S) - The figure shows components of fuel reformer. (Drawing includes non-English language text).    Reformer (1)    Heat exchanger (2)    Carbon monoxide removal apparatus (3,4)