To suppress a screw thread of a female screw and formed tooth provided in a screwing part from being deformed in an early stage of forming the female screw when the female screw is formed on the inner peripheral surface of a lower hole by screwing a screwing part of a tap into the lower hole of a nut blank.

An inclining angle at a biting part 24 in the nut blank 21 is defined as 15, thereby reducing the biting to the biting part 24 with the formed tooth 12 in a rolling tap 11 in an early stage of the formation for female screw into the nut blank 21 using the rolling tap 11. Thus, a torque acted in the rotating direction of the rolling tap 11 for forming the female screw in the biting part 24 is suppressed from becoming large with the formed tooth 12, and the formed tooth 12 of the rotated tap 11 is suppressed from being strongly pushed in the axial direction to the biting part 24 bited as the state. Therefore, the screw thread and the formed tooth 12 are suppressed from being deformed caused by the pushing.