A backlight unit includes: a light source unit which outputs coherent light; a first reflection unit including a parabolic mirror; a second reflection unit facing the first reflection unit and including a flat mirror; and a holographic optical element which changes a path of incident light, where a reflection surface of the second reflection unit forms an acute angle with an light incident surface of the holographic optical element, and the coherent light output from the light source unit sequentially passes the first reflection unit, the second reflection unit, and the holographic optical element.