A coherent backlight unit and a 3D image display device including the coherent backlight unit are provided. The coherent backlight unit includes a light source, a light guide plate, a first diffraction grating, a second diffraction grating, and a reflective optical element. The light source irradiates a coherent light and the reflective optical element reflects the coherent light that is propagating toward one of the side surfaces from the inner side of the light guide plate toward the inner side of the light guide plate.