A composite green sheet which comprises a green sheet including an inorganic powder and an organic binder, and a conductive layer formed thereon is prepared; the composite green sheet is then placed on a supporting surface of a base member containing a liquid; and the composite green sheet is irradiated with a laser light so as to form a through hole according to need, and thereby to evaporate the liquid contained in the base member so that waste remaining on the inner wall surface of the through hole is cleaned and removed. As a result, the conductive layer is exposed at the inner wall of the through hole penetrating the conductive layer.