PURPOSE: To raise the concentration. of heavy hydrogen in a synthesis gas by reevaporating process condensed water discharged from a hydrocarbon reforming process and a carbon monoxide converting process and by supplying the resulting steam to the reforming process.
CONSTITUTION: In steam reforming of naphtha, naphtha 1 passed through a desulfurization process enters reforming process A together with steam 5, and it is decomposed and taken out as synthesis gas 2 through carbon monoxide converting process B and decarboxylation process C. Condensed water 3 discharged from processes B, C is converted into hot steam 4 by heater D, and steam 4 is supplied to process A again. By continuously performing steam reforming of naphtha 1 the ratio of heavy hydrogen in gas 2 is raised. In addition, by this method the amount of steam 5 supplied is reduced.