PURPOSE: To surely remove scale by injecting a fluid on the inner peripheral surface of a pipe stock which is reduced on the inlet side of a skew rolling roll while a plug is inserted into the pipe stock, the above-mentioned fluid is injected on the inner peripheral surface from the tip of the plug and a lubricant is injected from the rear part of the above-mentioned plug.

CONSTITUTION: The reduced part P1 of the pipe stock P which is reduced on the inlet side of the skew rolling roll 1 is made in the same as if it is bending worked. Scale on the inner peripheral surface of the pipe stock in the reduced part is generated with a lot of crack and made into such a state that scale is easily stripped. On the inner peripheral surface of the pipe stock in the reduced part just after the pipe stock is reduced on the inlet side of the skew rolling roll, the fluid of which the momentum is comparatively small and cooling effect is small is injected from the tip of the plug 2. Compressed air is injected from the tip of a nozzle member 2a which is provided at the tip of this plug 2 and the lubricant is injected from the intermediate part of the nozzle member 2a. In this way, scale on the inner peripheral surface of the pipe stock can be surely removed and the decline of the life of plug can be prevented.