PURPOSE: To obtain high working accuracy by working a deep part of a hole of a work to a hemispherical shape at the time of preforming, forming continuously an opening end of this hole and a connecting part of the hemispherical deep part, and setting a distance between this connecting part and a stepped part brought to upsetting the specific folds or above of the diameter of the hemispherical bottom part.

CONSTITUTION: Preforming is executed by a cold forging device for preforming and on one end face of a work W brought to cutting, a hole 2 of prescribed depth is formed. Upsetting is executed by a cold forging device for upsetting and on the upper part of the work W, a tooth surface 3 is formed, a stepped part 3 is formed on its rear, and also, holes 3b, 3c are formed on the upper and the lower faces of the center in the axial direction. A distance L between a connecting line P of a deep part 2a of the hole 2 of the work W and the stepped part 3a is set to ≥0.4 fold of the diameter D of the deep part 2a. Piercing is executed by a cold forging device for piercing and a wall thickness portion (t) between the holes 3a, 3b is brought to blanking and a hollow part 4 is formed. In such a way, high working accuracy is obtained and a stable product can be supplied continuously.