PURPOSE: To improve the chipping and breakage resistance and facilitate regrinding by forming a thread on the flank of each of the front cutting edge and rear cutting edge in the course of a thread cutting tool, and making the lead larger than the lead of a worked thread.

CONSTITUTION: A chip 1 has the same edge form 1 as the torsion form of a worked screw, the flank 6 of the front cutting edge 3 in the course of a cutting tool has a lead X times the lead of the worked screw in the same direction as said lead, and the flank 7 of the rear cutting edge 4 in the course of the cutting tool also has a lead X times the lead of the screw in the reverse direction. A nose part 8 is situated on the line C from the top end T to the terminal end E, and the edge form when the chip 1 is cut on the radial plane passing the tool shaft center can reproduce the original form, though the cutting edge length is slightly shortened. Even if the lead ratio X is 1, the regrinding is made easy, but at less than 2, the rubbing of the ground surface and the flanks results in the action of an abnormal grinding force, so that X is preferably not less than 2.