PURPOSE: To improve a work surface in quality as well as to decrease the occurrence of defective threading, by performing thread cutting an a cutting speed commensurate to tensile strength in a work material.

CONSTITUTION: Cutting speed V is expressed as a linear functional of tensile strength Ts as in the following expression, that is, V=f(Ts)=A×Ts+B. Here, A and B are of a constant, and as for a steel material containing s of below 0.015%, it is recognized to be A=-2.25 and B=304 like that. In this connection, these values of said A and B are reasonable even in regard to steel materials for tubing use an that it is unnecessary to change a functional from of the above-mentioned expression due to the cutting distribution, tool form, tool material, etc.