A semiconductor device of an embodiment includes a p-type first diamond semiconductor layer, a p-type second diamond semiconductor layer disposed on the first diamond semiconductor layer, a plurality of n-type third diamond semiconductor layers disposed on the second diamond semiconductor layer, and a first electrode disposed on the second diamond semiconductor and the third diamond semiconductor layers. The p-type second diamond semiconductor layer has a p-type impurity concentration lower than a p-type impurity concentration of the first diamond semiconductor layer and has oxygen-terminated surfaces. Each of the third diamond semiconductor layers has an oxygen-terminated surface. The first electrode forms first junctions between the first electrode and the second diamond semiconductor. The first electrode forms second junctions between the first electrode and the third diamond semiconductor layers. The first junctions and the second junctions are Schottky junctions.