PURPOSE: To simplify a roll elevating/lowering mechanism and to contrive the space-saving and improved rigidity of the main body of a forming-roll supporting machine by rotating forming rolls having high cross angles in the converging direction in the advancing direction of a tube on the pass line on their axes, revolving the tube by contact friction between the forming rolls and the tube and continuously executing skew rolling.

CONSTITUTION: A skew rolling device has a roll working part in which three forming rolls 2a, 2b, 2c are uniformly arranged at 120° intervals around the passes line 1. The forming roll is composed of two truncated cones, the half conical angle of the reeling part at the tip is equal to the cross angle α and the 2nd succeeding half conical angle is smaller than the cross angle. The axes 3a, 3b, 3c of rotation of the rolls are inclined at the cross angle α=45° in the converging direction in the advancing direction of the tube on the pass line 1. During forming the tube, the inside diameter of tube is restrained with a plug which is attached to the tip of a mandrel shaft which is held with a mandrel holding device 5 whose rear end part is held and which is freely rotatable.