An opening is formed in resin 20 by a laser beam so that a via hole is formed. At this time, copper foil 22, the thickness of which is reduced (to 3 .mu.m) by performing etching to lower the thermal conductivity is used as a conformal mask. Therefore, an opening 20a can be formed in the resin 20 if the number of irradiation of pulse-shape laser beam is reduced. Therefore, occurrence of undercut of the resin 20 which forms an interlayer insulating resin layer can be prevented. Thus, the reliability of the connection of the via holes can be improved.