Abstract: A decoking apparatus suitable for use on a reaction vessel for the thermal cracking of heavy petroleum oils the apparatus essentially including a rotatable main injection pipe to be disposed in the reaction vessel and having a multitude of jet nozzles along its length, and a second or auxiliary injection pipe positioned in the proximity of the main injection pipe to inject a scrubbing liquid over the outer peripheral walls of the main injection pipe to prevent deposition of coke. The main and auxiliary injection pipes are both supplied with heavy petroleum oil to remove the coke deposition from the reactor wall by the heavy petroleum oil jets from the main injection pipe while wetting the exterior of the main pipe with the heavy petroleum oil injected by the auxiliary injection pipe.