PURPOSE: To permit the machined diameter of a screw to be varied freely by installing a plurality of threading tools at equal intervals in the circumferential direction, relative to a tool body part, being shifted in succession in the direction of movement.

CONSTITUTION: Cutting tools 3 and 3' are fixed at equal intervals in the circumferential direction, permitting mounting and demounting onto a tool body part 1. Said cutting tool 3 is slidable in the radial direction in the cutting groove 1 on the body part 1, and fixed by bolts 5. The axial discrepancy of position between cutting tools, namely the cutting feed amount between the contiguous cutting tools is determined according to a calculation equation, and also the relationship between the radial positions of cutting tools is determined by making calculation on the cutting amounts. Thus, the cut area by each cutting tool is made equal, and each cutting force is mutually balanced.