PURPOSE: To make able the automatic correcting of high efficiency and high accuracy by detecting the curved part by rotating the material, measuring the material diameter, measuring the curved volume with the motion of the moving pushing tool, advancing the pushing tool, giving the plastic strain and correcting.

CONSTITUTION: By chucking the material 6 with the chuck 2a, 2b of the rotation device and rotating, the curved part is detected and the diameter D of material is measured. This rotation device is arranged at the material feeding part. The curved volume l is measured with the position detecting instrument 11 by moving the moving pushing tool 9. Further, the correcting part 4 to advance the pushing tool 9 and to give the required plastic strain is set. In such a way, the visual confirmation and the hand operation can be made unnecessary.