NOVELTY - A hydrocarbon oil (C) contains sulfur content of 0.3 mass ppm or lower, aromatic (aroma) content of 10 vol.% or less and naphthene content of 40 vol.% or less. The hydrocarbon oil has ratio (H/C) of hydrogen element and carbon element of 2 or more, flash point of 40degreesC or more, 90 vol.% distillation temperature of 250degreesC or less and steam reforming index of 20 or more.    USE - Used as fuel for burner, which is used as heat source of steam reforming-type modifier, and for production of hydrogen in hydrogen production system (claimed).    ADVANTAGE - The hydrocarbon oil enables efficient production of hydrogen without generation of nitrogen oxide.    DETAILED DESCRIPTION - A hydrocarbon oil contains sulfur content of 0.3 mass ppm or lower, aromatic (aroma) content of 10 vol.% or less and naphthene content of 40 vol.% or less. The hydrocarbon oil has ratio (H/C) of hydrogen element and carbon element of 2 or more, flash point of 40degreesC or more, 90 vol.% distillation temperature of 250degreesC or less and steam reforming index of 20 or more. The steam reforming index is calculated from an equation: 20xH/C-aroma-10xdistillation temperature. The distillation temperature is calculated from an equation: (IBP+T50+2xT95-700)/T50, where IBP is initial boiling point of hydrocarbon oil, T50 is 50 vol.% distillation temperature and T95 is 95 vol.% distillation temperature. An INDEPENDENT CLAIM is included for hydrogen production system comprising steam-reforming-type modifier. The mixed gas of hydrocarbon oil and steam, and water (D) are reacted in the presence of modification catalyst containing group VIII element of periodic table as active metal, at 400-10000degreesC and hydrogen is produced. The blend ratio of water and hydrocarbon oil is 1-5 mol/mol.    DESCRIPTION OF DRAWING(S) - The figure shows the flowchart of steam reforming-type modifier. (Drawing includes non-English language text).    hydrocarbon oil (C)    water (D)    burner (G)    steam (I)    reformer (J)