PURPOSE: To roll a fine seamless steel tube without flaw on the inner surface by measuring piercing efficiency under rolling and knurling roll surfaces so as to make that measured value not less than a preliminarily determined value.

CONSTITUTION: The higher piercing efficiency E is, the better productivity is. But inversely inner surface flaw has a tendency to be easy generated on the inner surface when the efficiency becomes ≤40% in the case of the skew rolling mill. This is because the number of idle spinning of inner surface of the roll 1 is increased. In this way, it is better to make that the max. roughness Rmax. ≥30μm to secure the piercing efficiency E at the value of ≥40%. Therefore, by always measuring the piercing efficiency E and performing knurling on the roll surface with a knurling device 3 when the lowest limit is made to reach the vicinity of 40%, the fine seamless steel tube without flaw on the inner surface can be rolled.