To enable further accurate and easy formation of a female screw.

In a thread forming tool 1 to form a female screw in a prepared hole 14 formed in a workpiece 13, a tap 2 to cut the thread of a female screw in the inside surface of the prepared hole 14 by pressing in the prepared hole 14 with a screw part, where a thread is formed in the peripheral surface of a rod material, rotated, is movably held along a rotary central axis when a female screw is cut by a cap 3. With the end face of an opening end 11 pressed against the workpiece 13, and by guiding the screw part 4 to the center of the prepared hole 14 with the screw part 4 of the tap 2 approximately coinciding with a rotary central axis coinciding with the central axis of the prepared hole 14, the rotary central axis of a tap is approximately coincides with the central axis of the prepared hole, the thread of the female screw can be cut in the prepared hole 14 by the tap 2, and thus, the female screw can be formed further accurately and easily.