PURPOSE: To conduct drilling operation accurately and efficiently by automati cally detecting the position of a reference guide mark for drilling affixed to the internal layer of a multilayer printed board by using X-rays.

CONSTITUTION: X-rays are applied from the upper section of a multilayer printed board 8, and the pattern image of transmitted light transmitted through a guide mark 3 is detected by an ITV camera 9 for X-rays. An image-intensified signal is clipped at a variable threshold by a clipping circuit 12, the enhancement processing of an X-ray image is conducted by an analog multiplier 13, and the obtained value is clipped at a specified threshold by a clipping circuit 14. A pattern image is changed into a binary picture element by a binary-coding circuit 15, a reference pattern stored in a reference pattern memory 17 and a pattern cut out by an image cut-out memory 16 are made to correspond, the conformity of exclusive OR is obtained by a comparison circuit 18, and added by an adder 19, and the coordinates of the central position of the pattern of a picture element having highest conformity is acquired by a maximum value detector 22. Drilling is performed on the basis of the coordinates.