In FIG. 2, a diameter Dd of drilling edges of a drill is selected smaller than a valley diameter d1 of a male thread of a tap portion, and an inner diameter D1 of a female thread is defined by the valley diameter d1 of the male thread. Therefore, a predetermined radial gap is formed between the male thread 30 formed by the tap portion and the drill. When a tap with the drill is reversely rotated to be pulled out from the female thread after cutting the female thread, the tap released from the restriction may deflectively displace in the diameter direction upon pulling out the male thread of the tap portion from the female thread. However, the drill hardly contacts with the female thread, so that the occurrence of the defective product due to the contact is suppressed.