PURPOSE: To eliminate the bonding operation of a releasing film, and thereby automate counterbore processing as well as to enhance both processing accuracy and uniformity by detecting whether or not there exists the boundary surface of an intermediate layer within a work piece by means of ultrasonic inspection.

CONSTITUTION: When counterboring is processed, and when the intermediate layer Wa of a multi-layer printed wiring board W is cut, the boundary surface of the layer is lost, and ultrasonic waves transmitted to the multi-layer printed wiring board W from an ultrasonic sensor 8 are not reflected, so that no reflection of ultrasonic waves from the multi-layer printed wiring board W is thereby detected by the ultrasonic sensor 8. This thereby proves that counterbore processing reaches the intermediate layer Wa within the multi-layer printed wiring board W. At this point, a feed motor 4 is suspended by a feed motor control device 10 based on the signal from a measuring device 9, the descending drive of a movable stand 3 is then suspended, so that processing is thereby suspended in a state that counterbore processing has reached the intermediate layer Wa within the multi-layer printed wiring board W.