In one embodiment, a diamond semiconductor device includes a first diamond semiconductor layer of a first conductivity type which has a main surface, a second diamond semiconductor layer of an i-type or a second conductivity type which is provided on the main surface of the first diamond semiconductor layer, and has a first side surface with a plane orientation of a {111} plane, a third diamond semiconductor layer of the first conductivity type which is provided on the first side surface, and a fourth diamond semiconductor layer of the second conductivity type which is provided on the main surface of the first diamond semiconductor layer and on a side surface of the second diamond semiconductor layer at a side opposite to a side of the third diamond semiconductor layer.