PURPOSE: To aim at efficiently and accurately adjusting the tip of a tool, by operating a rotary means for rotating outer and inner spindles to change the rotational phases of outer and inner spindles under automatic control.

CONSTITUTION: The centers of outer and inner spindles 2, 3 are made to be eccentric by an eccentricity (e), and a tool fitting hole 4 is also made to be eccentric with the inner main spindle 3 by an eccentricity (e). Accordingly, the cut depth (a) is changed in accordance with the following equation, a=e(1-cosθ), where θ is a rotational phase angle. When a motor 10 rotates a thread shaft 6, the outer spindle 2 and the inner spindle 3 are rotated reversely to each other, due to the meshing between gears 5a, 5b having reversely twisted angles and gears 7, 9 so that the rotational phase angle θ is determined. When a desired cut-depth is inputted at a control means 14, the rotational phase angle is calculated, and a motor rotating angle (θ) giving θ is delivered to a controller 13. The controller 13 rotates the motor 10 in accordance with the rotational angle (θ). Thus, the tip of a tool may be automatically moved to a predetermined position, and therefore, it is possible to efficiently and accurately adjust the position of the tool tip.