A semiconductor device includes a semiconductor package having a differential signal terminal pair, and a wiring board. The wiring board includes a first and a second signal transmission line and a reference potential plane. The first and the second signal transmission line is formed in a first conductive layer and connected to the differential signal terminal pair. The reference potential plane includes a conductive pattern formed in a different conductive layer from the first conductive layer. The conductive pattern includes a first and a second region overlapped with the first and the second signal transmission line in plan view, respectively. The conductive pattern has a plurality of openings in the first and the second region. An area of a first conductive portion of the reference potential plane in the first region becomes equal to an area of a second conductive portion of the reference potential plane in the second region.