PURPOSE: To execute highly expanding/rolling without hindering yield and productivity by satisfying an equation which shows that there is the optimum range in the relationship between the ratio of a roll diameter and the outside diameter of a tube stock and tube expanding rate.
CONSTITUTION: A pair of cone type rolling rolls 31A, 31B are inclined and arranged at a fixed lead angle β to the pass line. Simultaneously, the rolls are intersected and arranged at a crossing angle to the pass line so as to have a face angle α1 on the inlet side and face angle α2 on the outlet side to the pass line and a hollow tube stock is highly expanded. At this time, the next inequality of the relationship between the ratio Dr/Di of the roll diameter Dr in the gorge part to the outside diameter Di of the tube stock on the inlet side and a tube expanding rate Er is satisfied. 1/2Er+2.9≤Dr/Di≤-1/2Er+10.1. Where, Er=(Do-Di)/Di, Do: the outside diameter of tube on the outlet side. In this way, the defective bit, ing and defective rear passing-out of a material to be rolled and hollow breaking due to flaring are evaded, the freedom of rolling setting is raised and high expansion and rolling of tube are executed without hindering yield and productivity.