As shown in FIG. 1, the rake face of the cutting edge 26 of the shank crests 24b of the complete crest 24, chamfer 30 is provided at the chamfer height Hmen within the range of 15% to 100% of the thread height Hneji because it is, by debris outright along the chamfered 30 it is easily pushed out to the outer peripheral side, with itself is suppressed biting of the chip, since the edge strength cut by the chamfer 30 is increased, biting of the chip blade chipping or breakage can be effectively prevented. Further, complete and 5 mountain following chamfered side ridges 24a than one pile of the distal end side of the crest 24, which has a restore full cutting edge shape, shank crest chamfer 30 is provided because 24b except also cutting edge 26 portion which is the original thread profile, good guiding action across the full crest 24 is obtained, it can be machined internal thread with high machining accuracy.