PURPOSE: To prevent the seizure of rolls or tripping of driving and to prevent the generation of a defect in thickness and diameter by actually measuring the shell speed just before a shell is bitten in a sizer and controlling the roll speed of the sizer in accordance with this actually measured shell speed.

CONSTITUTION: The speed of a shell 5 on the outlet side of a mandrel mill is first determined and the speed of the shell 5 on the inlet side of the sizer is determined in accordance with the ratio of the rotating speeds of the mandrel mill 1 and the sizer 3. This ratio is set as the rotating speed of the respective motors M of the sizer 3 and is kept stored in an arithmetic unit 12. The shell 5 is rolled in this state by the mandrel mill 1. Whether the rolled shell 5 exists between the mandrel mill 1 and the sizer 3 or not is detected by a shell detecting sensor 10. The arithmetic unit 12 calculates the difference between this signal and the stored rotating speed of the sizer roll 4 as a reference to calculate the difference in the rotating speed from the sizer roll 4, determines the rotating speed of the sizer roll 4 and corrects the rotating speed of the driving motor M of the sizer roll 4.