PURPOSE: To offer a three-roll rolling mill which is incorporable into a two-roll rolling mill in a rolling mill with which a billet which consists of steel or nonferrous metal is rolled into a bar, wire rod, tubing and other long sized materials with circular or annular cross section.

CONSTITUTION: This rolling mill is constituted by arranging the roll arrangement so as to shift the phase by 60° with respect to the axis of rolling of material to be rolled, connecting a gear train and universal joints 18a, 18b to each of two three-roll rolling mills, making two the above-mentioned universal joints connectable to two roll driving shafts 11, 11a for driving a two-roll rolling mill and arranging a one-way clutch 37 into the gear train on the other side. The two-roll rolling mill and three-roll rolling mill become freely exchangeable, the rolling mill can be freely switched depending on the purpose and the freedom of operation is drastically improved.