PURPOSE: To prevent effectively caulking in a pipe system of the top of a reaction column such as a free boat part, etc., by adjusting the flow velocity and the height of the free boat, by making the concentration of coke particles jumping out from the reaction column higher than the given concentration.

CONSTITUTION: Coke particles are circulated between the two fluidized beds of a reaction column and a heating column, the coke particles are heated by the heating column, and heavy oil is decomposed thermally by the reaction column using the coke particles as a heating medium, to give an olefin. In the operation, the flow velocity of a free boat and the height of the free boat are adjusted, and the concentration of the coke particles jumping out from the reaction column is made ≥0.5kg/m2.

EFFECT: Improving extremely the caulking growth rate in the pipe system of the top of the reaction column and the operation efficiency of a device for thermal decomposition.