A semiconductor device including a plurality of units having identical structures, each unit includes: a drain electrode; a drift layer that includes a low concentration layer on the drain electrode and a reference concentration layer on the low concentration layer, a gate electrode on the reference concentration layer; a pair of source regions that are provided on an upper surface of the reference concentration layer and in the vicinity of both ends of the gate electrode; a pair of base regions that surround outer surfaces of the source regions; a source electrode electrically connected to the source regions and the base regions; and a pair of depletion-layer extension regions that are respectively provided under the base regions in the reference concentration region. Boundaries between the depletion-layer extension regions and the low concentration layer are positioned lower than a boundary between the reference concentration layer and the low concentration layer.