PURPOSE: To increase reaction efficiency and heat transfer performance and to enable the protection of combustion catalyst by dividing a catalytic combustion part into a combustion part of the bottom part and a combustion part surrounding the reactive tubes, selecting preliminarily the separatory liquid ratio of fuel and supplying the quantity of heat necessary for the reforming reaction.

CONSTITUTION: Fuel feeding flow rate is automatically adjusted with a flow rate control valve 24 and the fuel is burned in the condition of remarkably excess air by feeding the whole quantity of the air for combustion to a combustion part 21 of the bottom part and the combustion temp. can be maintained at suitable temp. The remaining fuel (50W80%) is divided and fed to the optional positions of 3W6 positions of a combustion part 22 surrounding the reactive tubes. The proportion of said divided feed is preliminarily preset by making coincident with the distribution of endothermic zone and adjusted with flow rate control valves 25a, 25b, 25c so that the proportion becomes this preset value. The above- mentioned respective flow rate control valves are allowed to coincide with the preliminarily programmed condition with a microcomputer 26 in accordance with the measured value of temp. of the produced gas fluid.