To attain required formability without changing rolls by having plural calibers along the direction of the axis of rotation on the same roll surface and forming plural roll grooves.

By dividing the range of combination of the whole of side rolls or cluster rolls into several stages and narrowing the range of combination of the same groove, it is possible to attain the required formability without changing the rolls for securing the stability of forming. The plural calibers are provided along the axial direction on the roll surface of a single side roll or a cluster roll. It is proper that the range of combination of each roll cliber is 1.5-2.0 in the outside diameter ratio of a product. As for the caliber, it is desirable to provide the caliber so as to cope with the formation of the steel tubes having various sizes within the range of combination. Even when a type such as plural pairs of the side rolls or the cluster rolls on which respectively different roll grooves are formed are piled up and, so-called, skewered, 4 the same effect is attained.