Disclosed is a method of forming a dummy gate in manufacturing a field effect transistor. The method includes a first process of exposing a workpiece having a polycrystalline silicon layer to plasma of HBr gas, and a second process of further exposing the workpiece to the plasma of HBr gas after the first process. The first process includes etching the polycrystalline silicon layer to form a dummy semiconductor part having a pair of side surfaces from the polycrystalline silicon layer, and forming a protection film based on a by-product of etching on the pair of side surfaces in such a manner that the thickness of the protection film becomes smaller toward a lower end of the dummy semiconductor part.