PURPOSE: To carry out the thermal cracking of heavy oil in high economicity and operation stability, by using a fluidized-bed reactor containing coke particles as thermal medium, separating particles from the suspension liquid intruded into the treating apparatus, mixing the particles with heavy oil, and recycling the mixture to the reactor.

CONSTITUTION: The combustion gas 25 is subjected to heat exchange with coke particles in the heating tower, and introduced together with steam 24 into the cyclone 4, in which the coke is separated and removed to obtain exhaust gas 26. The cracked product obtained by the contact of the heavy oil 21 with coke in the reaction tower is introduced together with steam and coke into the cyclone 5 and the quencher 6, and fed to the fractionating tower 7 as the cracked product 31. In the above process, the suspension liquid composed of coke particles, solid carbonaceous particles and by-product oil and entering into the fractionating tower together with the cracked product is treated with the separator 10 to separate the coke particles and the solid carbonaceous particles from the suspension liquid, and the separated particles are mixed with the heavy oil and recycled to the fluidized bed reactor.