A metal layer 18 is sandwiched between insulating layers 14 and 20 so that required strength is maintained. Hence it follows that the thickness of a core substrate 30 can be reduced and, therefore, the thickness of a multi-layer printed circuit board can be reduced. Formation of non-penetrating openings 22 which reach the metal layer 18 in the insulating layers 14 and 20 is simply required. Therefore, small non-penetrating openings 22 can easily be formed by applying laser beams. Thus, through holes 36 each having a small diameter can be formed.