For suppressing decomposition of an organic group (for example, a CH₃ group) which is bonded to an Si atom of an organic SOG film for use in a flattening process at the time of an ashing process, there is provided a method comprising the steps of: forming an organic SOG layer directly on a lower wiring layer or on a predetermined film including a hillock protection layer which is formed on the lower wiring layer in advance; forming an upper wiring layer on the organic SOG layer without using an etching back process; forming a via hole through an etching process by using a patterned resist layer provided on the upper wiring layer as a mask; performing an ashing process with a plasma by making ions or radicals which are induced from oxygen gas as a main reactant, under an atmospheric pressure ranging from 0.01 Torr to 30.0 Torr; and filling said via hole with a conductive material so as to electrically connect the lower wiring layer to the upper wiring layer.