Provided is a back light unit (BLU) for holographic display. The BLU includes a first diffraction grating attached to a waveguide, a second diffraction grating that supplies light to the first diffraction grating, a light source that supplies light to the second diffraction grating, and a light reflection element that reflects light towards the first diffraction grating. The back light unit may further include a lens that is disposed in front of the waveguide. The light reflection element may be disposed parallel to a rear surface of the waveguide. The light reflection element may be inclined with respect to the rear surface of the waveguide. The light reflection element may be a mirror or a prism array.