An interposer to be interposed between a semiconductor chip to be mounted thereon and a packaging board has an interposer portion made of a semiconductor material and an interposer portion provided around the foregoing interposer portion integrally therewith. On both surfaces of the interposer portions, wiring patterns are formed via insulating layers. The wiring patterns are electrically connected via through holes formed at required positions in the interposer portions. The outer interposer portion is made of an insulator or a metal body. Further, external connection terminals are bonded to one surface of the interposer.