PURPOSE: To enlarge the rolling elongation on purpose to improve the accuracy of rolling, by giving an elliptical caliber to the roll at the position circumferentially 90 degrees shefted from that of adjoining rolling mill, and by rolling with plural sets of piercing mill whose mandrel is formed to have a section similar to the ellipse of the caliber.

CONSTITUTION: The thrust block 14 is advanced toward the rolling roll, and the mandrel 10 is inserted to the piercing portion of the pipe type raw material P, and only the mandrel is advanced; hereby, the mandrels 11, 12, 13, are settled at individual position corresponding to respective rolling roll 7, 8, 9. Then, the pipe type raw material P is advanced by the feed rolls 15, 15', and is fed to the first couple of rolling roll 7, 7, and is rolled. Next, the material P is fed to the second couple of rolling roll 8, 8, and the portion 38 being 90 degrees shifted from the first reduced portion 37 is pressed and rolled. Such rolling by the rolling roll given with the elliptical caliber and the mandrel of elliptical section is arbitrary times repeated at the reducing position shifted by 90 degrees in every rolling