A via hole is formed in a first cladding layer laminated on a wiring board. A conductive material is filled in the via hole so as to form a first conductor portion (a portion of a conductive via) having a mushroom-like shape projecting from a surface of the first cladding layer. Then, a second cladding layer is formed to cover the first conductor portion, the first cladding layer and a core layer, and a via hole is formed in the second cladding layer. A conductive material is filled in the via hole so as to form a second conductor portion (a remaining portion of the conductive via) connected to the first conductor portion.