Openings are disposed in an insulating layer to expose a conductor layer. A lower cladding layer is formed, and a resist layer is formed on an insulating layer and the lower cladding layer. Electrolytic plating is performed with using the conductor layer which is connected to the external, as an electrode, to fill openings passing through the lower cladding layer and the resist layer with Cu. The resist layer is removed away to form projections configured by the filled Cu. The projections are processed to have an inclined face. Au layers are formed on the inclined faces of the projection. A core layer and an upper cladding layer are stacked.