PURPOSE: To facilitate changing of roll phase angles for the purpose of suppressing angular bulging phenomena by transmitting revolutions and driving to one roll in a roll box, transmitting the revolutions to the other rolls by means of gears and fitting the roll box into housing.

CONSTITUTION: Rolls 7, 8, 9 in a roll box 21 are connected by means of gears 10, 11, and the rolls 7, 8, 9 are disposed by changing angles by 120° each in a circumferential direction within the plane perpendicular to the rolling direction. An intermediate shaft 15 is conducted in one roll 7, and the other intermediate rolls 14, 16 are connected to the shaft 15 by changing the angle about 30°C, whereby a reducing mill body 20 is formed. The box 21 is fitted into housing 25. A driving shaft 30 for transmitting driving power to the shaft 15 is provided in the housing 25, and a driving shaft 31 is provided to change the angle of the box 21 selectively. With such mechanism, the phase angle is changed easily in the plane perpendicular to the rolling direction, and the angular bulging phenomena of the inside surface of the drawn circular pipe is suppressed low.