A through-hole extending from an element-mounting surface to a back surface of a substrate is formed along an edge area of a bottom surface of the substrate. This through-hole is filled with a conductive resin paste directly into a quarter through-hole made in the substrate. This makes the conductive resin paste firmly adhere to the substrate preventing the conductive resin paste from being peeling off from the quarter through-hole even if the through-hole is reduced in diameter or has a cross-sectional configuration.