A printed wiring board includes an insulating substrate having a penetrating hole formed through the substrate, a first conductive pattern formed on first surface of the substrate, a second conductive pattern formed on second surface of the substrate on the opposite side of the first surface, and a through-hole conductor formed in the penetrating hole in the substrate such that the conductor is connecting the first conductive pattern on the first surface of the substrate and the second conductive pattern on the second surface of the substrate. The penetrating hole has a first opening portion opening on the first surface of the substrate, a second opening portion opening on the second surface of the substrate and a third opening portion connecting the first and second opening portions, and the third opening portion has the maximum diameter which is greater than the minimum diameters of the first and second opening portions.