To provide a female screw machining tool and a method for forming the female screw by which the machining resistance in the case of forming a multi-stripe female screw can be reduced.

The female screw 24 can be formed on the inner peripheral surface of a hole 62 by screwing in from the one side to the other side and from the other side to the one side to the hole 62 with the female screw machining tool 51. In the other words, the formation of the female screw 24 on the inner peripheral surface of the hole 62 is realized with a plurality of screwing-in times to the hole 62 with the female screw machining tool 51. Therefore, the number of screw-threads 53 in the female screw machining tool 51 is made less than the number of the female screw 24 which are scheduled to form in the inner peripheral surface of the hole 62, and the number of the female screw 24 formed at one time of screwing-in to the hole 62 with the tool 51 can be reduced, and then the machining resistance in the formation of the female screw 24 every time of the screwing-in of the female screw machining tool 51 can be reduced.