PURPOSE: To minimize the quantity of heat input to a mandrel to be inserted into the shell of a material to be rolled, and continuously roll a seamless pipe of superior durability by controlling the moving speed and moving distance in the rolling direction of said mandrel.

CONSTITUTION: A shell 21 of a material to be rolled is fed into between rolls 10W 17, and a mandrel 22 is inserted into the shell 21. The respective rolls 10W17 are disposed with alternate inclinations of 45°. The mandrel 22 is controlled of its speed and position freely by a driving device having a rack 23 and a pinion 24. The moving distance of this mandrel 22 is set not at the distance D between adjacent rolls for finish rolling but at less than the distance E up to the next roll, and the times when part of the mandrel 22 contacts with the rolls via the shell 21 are limited to twice or less. Thereby, the quantity of heat input to the mandrel 22 is reduced to less than a constant value.