A method of manufacturing a semiconductor device includes processes of forming a gate electrode, a source electrode, and a drain electrode on a nitride semiconductor layer, forming an insulating film including, on a surface thereof, a step that covers the gate electrode and reflects a shape of the gate electrode, and a flat portion, forming a mask on the insulating film, forming an opening in the mask, the opening including a shape in which a side surface of the step is located on an inner side of the opening and an upper surface end portion of the gate electrode is located on an outer side of the opening, and having an overhang shape extending in a depth direction, and forming a field plate extending from a side surface of the step to the flat portion using the mask.