To provide a method and apparatus for modifying heavy oil to an improved level of modification such as obtaining low viscosity modified oil from the heavy oil comprising a reaction step obtaining emulsion by carrying out a supercritical water reaction or subcritical water reaction (supercritical water reaction or the like) to the heavy oil.

The invention relates to the method and apparatus for modifying heavy oil to the modified oil having reduced viscosity comprises a process of pressure feeding the heavy oil and water for supercritical water reaction or the like with regulated pH of 7.5-12 by adding a pH adjuster, to a continuous reaction vessel 16, and forming the emulsion by the supercritical water reaction or the like.