TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Device: The hydrogen-production system comprises a water-gas-shift-reaction unit, steam-reforming type modifier or self-thermal-reforming type modifier. The water-gas-shift-reaction unit obtains carbon dioxide and hydrogen from reformed gas containing 1-80 mol/mol of carbon monoxide and water vapor, at a reaction temperature of 100-600degreesC, in the presence of a catalyst containing IB group, IIB group, VI group and/or VIII group metal as an active metal. The steam-reforming type modifier generates product having hydrogen as main component, by reacting water, hydrocarbon oil and mixed gas of hydrocarbon oil and water vapor at a blend ratio of 1-5 mol/mol, at a reaction temperature of 400-1000degreesC, in the presence of modification catalyst containing VIII group element as an active metal. The self-thermal-reforming type modifier generates product containing hydrogen as main component, by reacting water, hydrocarbon oil, mixed gas of hydrocarbon oil, water vapor and air at a blend ratio of 0.5-5 mol/mol and oxygen and hydrocarbon oil at a blend ratio of 0.1-0.5 mol/mol, at 400-1000degreesC, in the presence of modification catalyst containing VIII group element as an active metal. The hydrogen-production system comprises desulfurizer, modifier and carbon monoxide purification unit.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The hydrocarbon oil has 95 volume% distillation temperature of 240degreesC or less, and difference of 95 volume% distillation temperature and initial boiling point of 50degreesC or less. The molar ratio of carbon and hydrogen in hydrocarbon oil, is 1.95 or more. After extracting linear saturated hydrocarbon, the hydrocarbon oil is fractionated. Hydrodesulfurization processing of the raw material oil, is performed at 250-310degreesC. After fractionating, 60 volume% or more of light oil portion is mixed with the hydrocarbon which is subjected to hydrodesulfurization or removal of light oil portion or linear saturated hydrocarbon. EXAMPLE - A hydrocarbon mixture having initial boiling point of 155.5degreesC, 90 volume% distillation temperature of 234degreesC, aromatic content of 11.6 volume%, linear saturated-hydrocarbon content of 40 volume%, 10-15C linear saturated-hydrocarbon content of 31.4 volume% and sulfur content of 120 mass ppm, was subjected to hydrodesulfurization processing, at hydrogen pressure of 8.5 MPa, LHSV of 1.54 hour-1, hydrogen/hydrocarbon capacitance ratio of 0.26 and reaction temperature of 279-302degreesC, using nickel-type catalyst. 30 volume% of light oil was removed from the obtained hydrodesulfurization processed oil, followed by removing 28 volume% of linear saturated hydrocarbons using zeolite at 2 MPa. The obtained hydrocarbon oil had initial boiling point of 188degreesC, 50 volume% distillation temperature of 212.5degreesC, 90 volume% distillation temperature of 235degreesC, sulfur content of 0.16 mass ppm and density of 0.812 g/cm3.