PURPOSE: To make the outside diameter and wall thickness of a pipe uniform with high accuracy by driving always the two top stands alone among roll stands in which a hollow blank pipe inserted therein with a mandrel bar is bitten without driving the other stands in the stage of rolling said blank material.

CONSTITUTION: Plural roll stands are disposed, and the bottom directions of the caliber grooves of caliber rolls 17 in the stands are arranged approximately orthognally with each other to constitute a mandrel mill 15, wherein the respective rolls 17 are driven by independent motors 3 and speed reducers 32. A hollow blank pipe 14 inserted therein with a mandrel bar 16 is passed in the 1st stand of the mill 15 and is subjected to the operation of opening the clutch 33 of the stand (i-2) upon knowing of the top end thereof in the succeeding stand (i), i.e. the elongation by driving always the two top stands alone among the stands in which the pipe 14 is bitten without driving the other stands. The generation of a stomach part in the rolling with the mandrel mill is thus prevented.