A board 20 is provided with a Cu film 30 as a conformal mask, in which are formed a register mark 30b and an opening 3a through which a via hole is formed. A camera senses this register mark 30b so that the position of the board 30 is determined. A laser beam is directed to the approximate position of the opening 30a, so that the opening 26a through which the via hole is drilled is formed. The accuracy of the position of the opening of the via hole depends on the accuracy of the position of the opening 30a in the Cu film 30 as the conformal mask. Therefore, the via hole can be formed at an adequate position despite the low accuracy of the position for laser irradiation.