The present invention is directed to improving an insulating property of a backing in which a lead array is buried. The method includes a coating forming process, in which insulating coatings are formed with respect to at least a plurality of lead rows included in a plurality of lead frames; after the forming of the insulating coatings, a plate manufacturing process, in which a plurality of backing plates are manufactured by pouring a backing material towards a lead row in each of the plurality of lead frames so that the lead row and the backing material are integrated with each other; and a laminating process, in which the plurality of backing plates are laminated.