PURPOSE: To detect the position of a work with high accuracy even when a cutting tool is not a true circle.

CONSTITUTION: In state where a metallic cutting tool 7 is used and the cutting tool itself is rotated, the cutting tool 7 is moved toward a work 9 to be brought into contact with the work 9 based on command pulse signals from a controller at specified intervals. Electrical changes obtained in the contact state are seized as position signals to detect the position of contact of the cutting tool 7 with the work 9. The rotation period of the cutting tool 7 is made shorter than the specified interval of the command pulse signals, and the direction of rotation of the cutting tool 7 at the time of detecting the position of the work shall be opposite to the direction of rotation of the cutting tool 7 at the time of machining the work.