An illumination unit is provided including a coherent light source; a projection optical element which focuses a light beam emitted from the coherent light source onto a focal plane; and a holographic optical element interposed between the coherent light source and the projection optical element, and having an interference pattern formed thereon. The holographic optical element diffracts the light beam emitted from the coherent light source and emits the diffracted light to the projection optical element. Here, the interference pattern on the holographic optical element may have information that diffracts the light beam and thereby cancels a diffraction of the light beam due to an aberration of the projection optical element.