A semiconductor device comprises a semiconductor layer having a semiconductor integrated circuit, which is for processing an electrical signal, on a semiconductor substrate and an optical interconnect layer for transmitting an optical signal are joined. Control of modulation of the optical signal transmitted in the optical interconnect layer is performed by an electrical signal from the semiconductor layer, and an electrical signal generated by reception of light in the optical interconnect layer is transmitted to the semiconductor layer. The optical interconnect layer is disposed on the underside of the semiconductor substrate.