A field effect transistor includes a stacked body, a source electrode, a drain electrode, a gate electrode, a dielectric layer and a silicon nitride layer. The stacked layer has a heterojunction made of a nitride semiconductor. The source and drain electrodes are provided on a surface of the stacked body. The gate electrode is provided on the surface of the stacked body between the source and the drain electrodes, and has a field plate portion. The dielectric layer is provided so as to cover an intersection line of a first side surface of the gate electrode and the surface of the stacked body. The silicon nitride layer is provided so as to cover a region between the source electrode and the gate electrode and a region between the dielectric layer and the drain electrode. The field plate portion protrudes from the first side surface.