the drive shaft with use of a tachometer the rotation and up down shifting of the main injection pipe 71 are effected separately in normal operations but both may be effected simultaneously instead of shifting the injection pipe assembly by the drive assembly 50 it is possible to operate the piston cylinder by fluid pressure for example by moving the piston 67 up and down by controlling the pressures of steam to be admitted into the upper and lower steam chambers 96 and 93 it will be appreciated from the foregoing description that according to the present invention the coke deposit on the reactor interior is removed by the high pressure jets of hot heavy oil which is injected as a scrubbing liquid through the jet nozzles of the main injection pipe to allow continuously repeated cracking operation by the reactor while injecting through the auxiliary injection pipe a similar raw material over the exterior of the main injection pipe to wet and to preclude coke deposition on the main injection pipe in addition the rotation and up down shifting of the main nozzle pipe within the reactor enlarges the area which is covered by the jets of scrubbing liquid and ensures more perfect removal of the deposited coke during the rotation and up down shifts of the injection pipe assembly leakage of the gases and inflammable hot asphalt from within the reactor is completely prevented by the secure seals which are very simple in construction and which facilitate maintenance and inspection moreover in a case where the auxiliary injection pipe is helically wound around the main injection pipe it can easily adapt itself to the thermal expansion of the.