PURPOSE: To enable sure and firm fixation of a holder member to the main body of a holder, as well as accurate and fine adjustment of the radius of rotation of a bit.

CONSTITUTION: The hydropneumatic pressure in a pressure chamber 12 raised up to a prescribed value by a piston chamber 15 produces elestic deformation in the this part 9 of a sleeve member 6 and thereby a holder member 35 is held firmly and closely on the outer-peripheral surface of a column part 36. Fine adjustment of the radius of rotation of a bit is performed by loosening a presser-screw 31, reducing the hydropneumatic pressure in the pressure chamber 12, inserting a wrench into the hole 52 of a supporting shaft 47 and turning therewith a worm 45, and thereby changing the distance between the tip of the teeth of the bit 38 and the axial line of the rotational center of the main body 1 of a holder. Said adjustment can be performed by watching a scale 53 of a member 49.