PURPOSE: To perform a precise base drilling in a drilling mark position from an initial hole by reading the image of a standard mark to calculate the error of a position just before base drilling, and correcting the position of a drilling mark by this error.

CONSTITUTION: At the base drilling of a first hole, the position of a standard mark detected by a mark position detecting means 18 is recognized as a position just before base drilling, and the error between the position just before base drilling and the initial position is calculated by an error arithmetic means 19. At the base drilling, the position of a drilling mark P detected by the mark position detecting means 18 is corrected by this error, and a base drilling position is gained by a base drilling position gaining means 21. At the base drilling on and after the second hole, a drill 5 is separated from a base K until the hole bored just before can be judged, and the error between the position of the hole and the position of the drilling mark P is determined by the arithmetic means 19. The position of the next drilling mark P is corrected by this error, and the base drilling position is gained by the gaining means 21 to perform the base drilling on and after the second hole.