PURPOSE: To prevent the occurrence of chattering due to flection in a cutting tool in time of projecting it longer than a tool rest outward, by clamping the cutting tool by dint of strong pressure applied.

CONSTITUTION: An external thread B' is cut in an outer diameter as having a clearance of about 1/100W9/100mm in between with an inner diameter of a through hole 4 of a toolholder 3 at the rear of a cutting tool B, then a fixed nut N1 is screwed in this thread B' and inserted into the hole 4 in an accurately fitting manner. Screwing a clamping nut N2 in an end part of the projecting external thread B' in advance, the nut N1 and the fixed nut N2 both are clamped in a nipping-hold direction of the toolholder 3, and the cutting tool B is forcibly clamped with pressure. For this reason, even if the cutting tool is projected outward longer than a tool rest, the occurrence of chattering is prevented.