An insulating substrate with metal foils on both sides thereof is
prepared, and a through hole that passes through the metal foils and
insulating substrate is formed. An electroless copper plating layer is
formed on an inner surface of the through hole and a surface of each of
the metal foils, followed by the formation of an electrolytic copper
plating layer on the overall surface of the electroless copper plating
layer. After removing the electrolytic copper plating layer except the
portions on the inner surface and a peripheral region of the through
hole, the metal foils are processed to form conductor patterns.