PURPOSE: To give a wide rolling size without rearranging a roll by rotating one side of a revolutional arm round the center of rotation of a planetary gear and the other side round the center of rotation of the solar gear.

CONSTITUTION: The solar gear composed of one external gear 25 and one internal gear 23 which have the same center of rotation are mounted on a power transmitting path from a power input axis 1 to a roll axis 6 in a three-roll type mill. Two planetary gears 24 engaging with the solar gears and a revolutional arm 26 where one is connected freely rotationally with the planetary gear 24 and the other rotates round the center of rotation of the solar gear are provided. The revolutional arm 26 is revolved by a power device. In this way, the peripheral speed of a 2nd pass roll corresponding to the reduction of the sectional area of material by rolling down of the 1st pass roll of three rolls can be selected freely.