PURPOSE: To carry out stable thermal cracking operation by thermally cracking heavy oil in a thermal cracking pass in the presence of steam while regulating the flow rate of a fluid in the pass to a specified value or above and feeding heavy oil from a simple tubular path in the forward direction of the fluid.
CONSTITUTION: Heavy oil contg. unvaporizable hydrocarbons with high mol.wt. is fed to a thermal cracking pass with feeders, and it is thermally cracked in the presence of steam while regulating the flow rate of a fluid in the pass to ≥10m/ sec. At least one of the feeders is a simple tubular path provided with only one path 2 only for ejecting heavy oil and no atomizing mechanism. Heavy oil is fed from the tubular path in the forward direction of the fluid. The tubular path is a trifurcate path having a path 1 for a gas for feeding heavy oil, a path 2 for feeding heavy oil and an opening 4 for spouting heavy oil, and heavy oil is fed at a rate equal to or higher than the flow rate of the fluid.