The suppression of coke adherence in a reaction vessel and cracked gas pipelines at the outlet of the reaction vessel and the prevention of plugging are designed by improving the dispersion of superheated steam inside the reaction vessel to preferable conditions, and pitch having more uniform and higher quality can be produced.A cracking process in which superheated steam is directly contacted with the heavy petroleum oil by blowing the superheated steam into the reaction vessel to be thermally cracked to oil fractions and pitch, wherein the superheated steam is blown into the reaction vessel from the bottom portion thereof so that the superheated steam generates rotational flow around the central axis of the shell of the reaction vessel.