PURPOSE: To detect exactly and automatically a center point in relation to positioning marks having various dimensions, by finding an area center within a standard circle after setting automatically a standard circle having a suitable dimension matching a mark dimension.

CONSTITUTION: Gradation change points 71a, 71b, 72a and 72b are found through image recognition of a positioning mark 70 on a printed circuit board W for finding a temporary center point 70a and a diameter of the mark 70 by a four- point search method. Next, a standard circle diameter is set by adding to the mark diameter an infinitesimal value set in advance for setting a standard circle having this diameter and a center of the temporary center point 70a. And then image processing is carried out within the standard circle, the mark 70 is divided into a plurality of partition and a center point 70b of the mark 70 is detected by finding a center point of gradation partitions indicating inside of the mark 70 for making a machining such as perforation.