A semiconductor device includes: a first insulating layer having an opening therethrough; a first wiring pattern disposed on the first insulating layer; an external connection terminal provided on a portion of the first wiring pattern exposed from the opening; a second insulating layer which covers the first wiring pattern and having via holes therethrough; a second wiring pattern disposed within the second insulating layer and electrically connected to the first wiring pattern via a conductive material filled in at least one of the via holes; a semiconductor element having an electrode thereon and mounted on the second insulating layer to be electrically connected to the first wiring pattern through the electrode disposed in at least one of the via holes; an underfill resin filled between the semiconductor element and the second insulating layer; and a sealing resin portion which seals the semiconductor element.