A semiconductor device comprises a substrate 1, a first n-type semiconductor layer 21′, a second n-type semiconductor layer 23, a p-type semiconductor layer 24, and a third n-type semiconductor layer 25′, wherein the first n-type semiconductor layer 21′, the second n-type semiconductor layer 23, the p-type semiconductor layer 24, and the third n-type semiconductor layer 25′ are laminated at the upper side of the substrate 1 in this order. The drain electrode 13 is in ohmic-contact with the first n-type semiconductor layer 21′ and the source electrode 12 is in ohmic-contact with the third n-type semiconductor layer 25′. A gate electrode 14 is arranged so as to fill an opening portion to be filled that extends from the third n-type semiconductor layer 25′ to the second n-type semiconductor layer 23, and the gate electrode 14 is in contact with the upper surface of the second n-type semiconductor layer 23, the side surfaces of the p-type semiconductor layer 24, and the side surfaces of the third n-type semiconductor layer 25′. The second n-type semiconductor layer 23 has composition that changes from the drain electrode 13 side toward the source electrode 12 side in the direction perpendicular to the plane of the substrate 1 and contains donor impurity.