A following margin of a following flank of an external thread is shifted more in a screw-rotation direction than a leading margin of a leading flank. Thus, during biting, the following margin bites first, thereby suppressing a force that pushes back a tap and reducing a tapping torque. Because margin portions can be disposed parallel to an axis of a thread forming tap, working is easy and manufacture can be performed inexpensively. When, in forming an external thread by grinding by a grinding abrasive, an inclination angle of the axis of a tap blank and an abrasive axis of the grinding abrasive is equal to a lead angle of the external thread, the margins are formed symmetrically across a root. On the other hand, if the inclination angle is made greater than the lead angle, the following margin can be shifted in the screw-rotation direction, enabling manufacture to be performed as easily and inexpensively as in a conventional case.