To simplify a tool exchange work and to shorten the tool exchange work time by moving a tool just a little along the axial direction of a hollow main spindle and its perpendicular direction.

When a cutting tool 19 is moved from a cutting tool holder 12 to the center of a hollow main spindle along the direction crossing at right angles with the axial direction of the hollow main spindle, a second stopper 36 is brought in contact with a key 18 in a rear side. In a point of the abutment, the positions of respective keys 17, 18 and respective clearance grooves are coincided with each other so that the cutting tool 19 changes its detachment direction. Sequentially, the cutting tool 19 is detached along the axial direction of the hollow main spindle on the cutting tool holder 12. As a result, the cutting tool 19 is attached/detached to/from the cutting tool holder 12 by moving just a little along the axial direction of the hollow main spindle 8 of the cutting tool holder 12 and its perpendicular direction so that the exchange work of the cutting tool 19 is facilitated and the cutting tool exchange work time can be shortened.