A nitride based semiconductor device includes: a substrate; a first buffer layer disposed on the substrate; a second buffer layer disposed on the first buffer layer; a third buffer layer disposed on the second buffer layer, the third buffer layer including an AlGaN-based nitride semiconductor; a fourth buffer layer disposed on the third buffer layer, the fourth buffer layer including a GaN-based nitride semiconductor; a barrier layer disposed on the fourth buffer layer, the barrier layer including an AlGaN-based nitride semiconductor; and a source electrode and a drain electrode, each disposed on the barrier layer, and a gate electrode disposed between the source electrode and the drain electrode, wherein the third buffer layer is subjected to lattice relaxation. There can be provided a nitride based semiconductor device capable of reducing a leakage current and improving breakdown capability.