A method of forming surface irregularities comprises preparing a GaN substrate; forming a mask on a surface of the GaN substrate, the mask defining a surface-irregularity formation region; and wet-etching portions of the surface of the GaN substrate by using the mask as an etching mask. The wet-etching of the GaN substrate is performed until the end of one surface of the GaN substrate to be formed by the wet-etching using the mask meets the end of another surface of the GaN substrate to be formed by the wet-etching using the mask, the another surface being adjacent to the one surface.