A heavy oil reforming system includes a reforming preheater raising the temperature of a mixed fluid comprising a high pressure heavy oil and high pressure steam up to a temperature for reforming reaction. The mixed fluid having been heated up to the temperature for reforming reaction is introduced into a reformer kept at the temperature for reforming reaction and thereby the heavy oil is reformed. This reforming system allows the attainment of a residence time of 1 to 10 min necessary for reforming in a uniform or nearly uniform temperature field, thereby implementing the manufacturing of reformed fuels from a large volume of heavy oil.