PURPOSE: To prevent the generation of buckling at the front end of a pipe material by keeping the roll gap of outer-diameter sizer wider than that of the sizer at working till the tip of pipe material arrives at the position between the rolls of sizer.

CONSTITUTION: A pipe material 1 is bitten in sequence by plural caliber rolls 14a, 14b arranged in the transferring direction of the material 11, to size its outer-diameter dimension. At that time, the rolls gap of each caliber roll is adjusted before starting the biting of the material 11 so that each average caliber diameter of the caliber rolls 14a, 14b is larger than the sizing dimension at each caliber roll. Each average caliber diameter is made equal to the sizing dimension at each roll by reducing the roll gas at each roll, at the time when the front tip of material 11 arrives at the bitting position where a compressive force, given to the material 11 in its axial direction from each of the caliber rolls 14a, 14b, becomes lower than the buckling load of material 11.