PURPOSE: To improve a reaction rate and conversion rate, etc., by adding an aq. ammonium sulfate soln. to heavy hydrocarbon raw material oil before crack ing and executing a pretreatment under prescribed conditions at the time of cracking the heavy hydrocarbon raw material oil by using an arom. hydrogen donor.

CONSTITUTION: The heavy hydrocarbon raw material oil (e.g. vacuum distillation residue), the hydrogenation condensed cyclic arom. compd. based hydrogen donor (e.g. tetralin) and the aq. ammonium sulfate soln. are introduced into a presoaking drum l. Presoaking is executed under the conditions of 316 to 482°C and a pressure of 206 to 13,900 kpa. The mixture is then introduced into a settler 2 where an aq. phase is settled and removed and the upper phase is transferred to an atm. pressure still 3. An overhead distillate is taken out. The liquid residue is then introduced into a bisbreaker 4 and is subjected to a bisbreaking operation or is introduced via a low-pressure furnace 5 into a delayed coker drum 6 where the oil is subjected to coking.