PURPOSE: To pick-up an image of a mark at one of positions which is most accurate in an image pick-up device, so as to enhance the position detecting accuracy.

CONSTITUTION: A setting step for detecting a point at which a strain is minimum in an X-ray camera provided to a drilling device, is carried out, and an optimum center position 6 is stored in memory. Accordingly, an image of a mark 61 set on a workpiece 4 such as a print substrate is picked up by image pick-up means (first step), and the center position 81a of the mark is detected in accordance with the result of the image pick-up (first position detecting step. Then, the image pick-up means is displaced so as to allow the stored optimum center position 83a to coincide with the center position 61a of the mark (image pick-up displacing step), and an image of the mark is again picked up by thus displaced image pick-up means (second image pick-up step). The center position of the mark is again detected in accordance with the result of the detection in the second image pick-up step (second position detecting step). Further, drilling means is displaced so as to be aligned with the center position of the mark redetected during the second position detecting step, for drilling the workpiece so as to form a hole 64.