PURPOSE: To improve the dimensional precision of a crank shaft by interposing, between an arm part and twist dies, a freely fittable jig tightly bonded to both of them for completely restraining the deformation of the arm part so as to carry out a twisting process.

CONSTITUTION: A jig 5 for restraining the arm part 4 of a crank shaft 1 is installed so that it may be fitted around the arm part 4 before the arm portion 4 is gripped by means of twist dies 12A, 12B respectively fixed on the insides of vertically divided halves of retainers 11A, 11B and interposed between the twist dies 12A, 12B and the arm portion 4. The retainers 11A, 11B and the twist dies 12A, 12B are rotated by specific angles by rotating a twist beam 13 in this state, thereby a journal portion 2 can be twisted and deformed. Thereby, only the journal part 2 can be reliably twisted and deformed, so that the dimensional precision of the crank shaft is improved.