PURPOSE: To improve the productivity by knurling the surface roughness of a roll in a specified range during rolling a seamless steel tube by a skew rolling mill.

CONSTITUTION: When the seamless steel tube is pierced or rolled by the skew rolling mill, the surface roughness of the roll 1 is measured by a measuring instrument 7 during rolling. As soon as the maximum surface roughness of the roll 1 falls, the maximum roughness is worked by a knurling device 6 to the range of 30-500μm. In this way, roll gripping force can be held in a proper range, the rolling chance of hard-to-work material can be magnified and the productivity for rolling can be improved.