Provided is a nitride semiconductor device including: a nitride semiconductor layer over a substrate wherein the nitride semiconductor has a two-dimensional electron gas (2DEG) channel inside; a drain electrode in ohmic contact with the nitride semiconductor layer; a source electrode in Schottky contact with the nitride semiconductor layer wherein the source electrode is spaced apart from the drain electrode; a floating guard ring in Schottky contact with the nitride semiconductor layer between the drain electrode and the source electrode; a dielectric layer formed on the nitride semiconductor layer between the drain electrode and the source electrode and on at least a portion of the source electrode wherein the dielectric layer is applied to the floating guard ring between the drain electrode and the source electrode; and a gate electrode formed on the dielectric layer to be spaced apart from the drain electrode, wherein a portion of the gate electrode is formed over a drain-side edge portion of the source electrode with the dielectric layer interposed therebetween, and a manufacturing method thereof.