PURPOSE: To prevent wear of the thread ridge of a machined female screw by applying chamfering on the part on the base end side, on the side where a lateral rake angle is positive, of the cutting edge of the chamfer of a spiral fluted tap and applying negative relieving of a frank continued to the cutting edge.

CONSTITUTION: In a spiral fluted tap 10, a thread ridge 16 formed such that an incomplete shape is gradually changed for a complete shape from the tip of a chamfer 20 toward a complete thread port 18 is peripherally divided by a helical flute 22, and a cutting edge 30 is formed to the one end part of the thread ridge 16 along the helical flute 22. In the spiral fluted tap 10, chamfering 52 is applied on a cutting edge 30, on the side where a lateral rake angle is positive, of the cutting edge 30 along flanks on both sides of the thread ridge 16 of the chamfer 20. Or, negative relieving is applied on a flank continued to the cutting edge 30. Thus, during a work to cut a female screw, cutting having reduced resistance and high precision is carried out.