In the upper surface of a metallic substrate, a region near the central part of the metallic substrate is surrounded by a rectangle having dotted sides electrically separate the interior and exterior of the rectangle. Each dot of the sides is formed of a pillared insulating resin that penetrates from the upper surface to the lower surface of the metallic substrate. Oxide films are so formed as to fill in the spaces between adjacent cylinders of insulating resins and the surrounding of the cylinders. That is, a separation layer is formed of the pillared insulating resins and the oxide films that fill up the spaces between the pillared insulating resins as well as their vicinities.