PURPOSE: To restrain a holder from deforming by the radial stress exerted on the holder tip, also to restrain the deflection of the holder and to make the holder length longer, by making the Young's modulus of the material of the holder larger than that of the material of a collet.

CONSTITUTION: In the case of a sintered hard alloy being used for the material of a holder 1 and steel for the material of a collet 2, and the Young's moduli of the both being taken as E1, E2, for example, E1 becomes about three times E2. The deformation of the holder 1 tip by the radial stress exerted as the reaction of the pressing force onto the taper face 4 of the holder 1 from the conical part 5 of the collet 2 is thus prevented, and the holder 1 can be formed in the same outer diameter at its barrel and tip parts. Also the deflection of the holder 1 is in inverse proportion to the Young' s modulus E1, it is restrained to 1/3 in case of being in the same shape, and the length of the holder can be made longer in the case of the deflection being made constant with the holder 1 in the same sectional shape.