A semiconductor device includes a supporting substrate, a conductive layer placed on the supporting substrate, and at least one group III nitride semiconductor layer placed on the conductive layer. Of the group III nitride semiconductor layers, a conductive-layer-neighboring group III nitride semiconductor layer has n type conductivity, dislocation density of at most 1×107 cm−2, and oxygen concentration of at most 5×1018 cm−3. Thus, an n-down type device having a semiconductor layer of high crystallinity can be provided.