A semiconductor device includes a substrate, an optical element, and a semiconductor element. The substrate includes a first region and a second region which are regions differing from each other. The optical element is formed in one of the first region and the second region. The electric element is formed in another of the first region and the second region. The first region includes a first insulating layer and a first semiconductor layer formed on the first insulating layer. The second region includes the first insulating layer, the first semiconductor layer, a second insulating layer formed on the first semiconductor layer, and a second semiconductor layer formed on the second insulating layer.