PURPOSE: To prevent a material to be rolled from being struck by a round- caliber roll on the non-driven side and from causing surface flaws of roll by slipping when the material to be rolled is bitten by a 4-roll type rolling mill provided with round kaliber rolls on the driving side and round kaliber rolls on the non-driven side which are oppositely arranged.

CONSTITUTION: Two opposed round-caliber rolls 1, 2 are rotationally driven by rolling torque, also a rotary impeller 17 extending along on the circomference in the vicinity of the outer periphery of side walls is arranged on the side walls of the round-kaliber rolls 3, 4 being the non-driven side, high-pressure water is jetted to this rotary impeller 17 from high-pressure water jetting nozzles 18 and, after preliminarily making the round-kaliber rolls 3, 4 in a state of revolution by that hydraulic driving-torque, the material 11 to be rolled is bitten.