PURPOSE: To convert a heavy hydrocarbon into light one at a low cost without strictly limiting the quality of feed by bringing a heavy crude oil feed into contact with a catalyst inside a liquid-phase reactor in a specific range of temperature, pressure, and retention time, bringing the product into contact with superheated steam, and then performing liquid-solid separation.

CONSTITUTION: Predetermined amounts of a heavy crude oil feed and a catalyst (granular catalyst containing metal alkoxide and/or phenoxide as active components) are introduced into a liquid-phase reactor, they are brought into contact with each other at a temp. about 350-500°C and under a pressure about 1-20 atmospheric pressures for a retention time of about 1 second to 5 minutes, the product obtained is brought into contact with superheated steam, preferably at a temp. about 300-500°C and under a pressure about 1 to 10 atmospheric pressures for a retention time of about 0.1 second to about 5 minutes, and the produced light hydrocarbon and coke are separated from each other by a liquid-solid separation system. Therefore at low capital investment and operation cost, the heavy crude oil and/or solid hydrocarbon can be converted into gasoline and other light fraction products without strictly limiting the quality of the feed and generating large amounts of C4 gas and coke.