PURPOSE: To apply an effective straightening by the use of a straightening machine with a small number of caliber rolls by coinciding the bending direction of the material to be straightened to the bending direction by each caliber roll and inserting each material into the pass line of the tubing rod.

CONSTITUTION: By supporting both ends of a material 1 to be straightened, at two points, namely at a lower pinch roll 3 and a receiving roll 4, the material is rotated due to gravity actuating on its centroid and imparted with an inserting posture such that the maximum bending direction is automatically set to a vertical direction. Then, the imparted inserting posture is retained by holding the material 1 between the upper and lower pinch rolls 3, 3; and by the rotating force of these pinch rolls 3, 3, the material 1 is inserted into the pass line of a tubing rod with the inserting posture retained. When the material 1 is inserted into the straightening machine 10, the upper and lower pinch rolls 3, 3 open. The material 1 is straightened by being imparted with the bend of each caliber roll 2 in the maximum bending direction for straightening by the straightening machine 10.