Provided is a semiconductor device according to an embodiment including an i-type or first-conductivity-type first diamond semiconductor layer having a first side surface, a second-conductivity-type second diamond semiconductor layer provided on the first diamond semiconductor layer and having a second side surface, a third diamond semiconductor layer being in contact with the first side surface and the second side surface, the third diamond semiconductor containing nitrogen, a first electrode electrically connected to the first diamond semiconductor layer, and a second electrode electrically connected to the second diamond semiconductor layer.