PURPOSE: To eliminate the variation in machining accuracy and prevent damage to a thin hole guide by forming the thin hole guide so that a driving means for the thin hole guide can be stopped by an output signal from a sensor when the thin hole guide is at a specified distance from a work.

CONSTITUTION: Before machining, a thin hole guide 2 is positioned at a working set-position by the rotation of a motor 10. Thus, when the thin hole guide 2 is sensored at a certain close distance from a work 3 by means of a sensor 11, a detecting device 12 is operated to detect that the thin hole guide 2 is at a constant distance from the work 3, and stop the rotation of the motor 10. In this instance, the machining position of the thin hole guide 2 is formed to be positioned by detecting its position by means of the sensor 11, so that the operability is improved, and damage to the thin hole guide due to misoperation is eliminated. The machining time can be shortened, and the variation in machining accuracy can be also reduced.