PURPOSE: To detect the position of a hole when the diameter of a drilling mark is smaller than the diameter of the hole by separating a drill from a base until the image of the hole can be judged.

CONSTITUTION: This device has a drilling mark size judging means 24 for judging whether a drilling mark P is larger than a hole or not. When the drilling mark P is larger than the hole, as a result of the judgment by the judging means 24, a first drill moving means 10 is selected by a switch 25 to separate a drill 5 from a base K by a relatively small distance. When the drilling mark P is smaller than the hole, a second drill moving means (10, 23) is selected to separate the drill 5 from the base K by a relatively large distance. Consequently, the position of the hole can be precisely detected regardless of the size of the drilling mark.