PURPOSE: To prevent coking trouble and thermally crack heavy oil, by thermally cracking the heavy oil to separate the oil into the vapor phase component and the liquid phase component, removing the spherulitic component from the liquid phase component, circulating the remainder through the thermal cracking process, and separating the vapor phase component into the heavy oil, light oil and gas component.

CONSTITUTION: Heavy oil is treated in a tubular furnace in the first thermal cracking step, and the cracking product is brought into contact with a gaseous heating medium in a continuous single reaction tank and thermally cracked to produce a thermally cracked oil and a thermally cracked gas as a vapor phase component and pitch containing dispersed spherulites as a liquid phase component in the second thermal cracking step. The resultant liquid phase component is then separated into a spherulitic pitch component and a matrix pitch component. The former is removed to circulate the latter through the second treatment step. The vapor phase component is separated into a heavy and light thermally cracked oil component and thermally cracked gas component, and the heavy component is thermally cracked in a tubular furnace to produce a light oil and tar in the third thermal cracking step. The tar is then circulated through the second treatment step.