PURPOSE: To automatically and precisely work by opening a circular long hole having a specific depth of axial center as a pre-hole and working with cold forging by using a punch having a top end of die square shaped part of a specific effective length.

CONSTITUTION: In an initial process, a round shaped long hole 4 of its diameter α, depth of effective axial center a+b+c shown in a inscribed circle of a cross- sectional square shape is opened along an axial center of a square shaped long hole 5 as a pre-hole on a blank 2. Then, a square round punch 9 attaching a die square shape part 9a corresponding to a square shaped long hole 5 whose cross-sectional surface is made in a diagonal distance D, an effective length (b) at its top end is pressed in a P direction along the axial center of the circular shaped long hole 4 till the position of the axial center depth b+c, and cold forged. The forming of the square shaped long hole 5 is finished with three workings such as the preliminary forming, the pre-hole opening and the cold forging, then, required post workings are advanced.