Disclosed are a method of manufacturing a field-effect transistor. The disclosed method includes: providing a semiconductor substrate; forming a source ohmic metal layer on one side of the semiconductor substrate; forming a drain ohmic metal layer on another side of the semiconductor substrate; forming a gate electrode between the source ohmic metal layer and the drain ohmic metal layer, on an upper portion of the semiconductor substrate; forming an insulating film on the semiconductor substrate's upper portion including the source ohmic metal layer, the drain ohmic metal layer and the gate electrode; and forming a plurality of field electrodes on an upper portion of the insulating film, wherein the insulating film below the respective field electrodes has different thicknesses.