Provided is a backlight unit including a light source emitting coherent light, a light guide plate propagating the light emitted by the light source, an input grating inputting the light from the light source into the light guide plate, an output grating provided on one surface of the light guide plate and diffracting light incident from inside the light guide plate and outputting the light, in a direction toward the outside of the light guide plate, a first recuperation element provided on one surface of the light guide plate and including a first recycle grating, and a second recuperation element provided opposite to the first recuperation element and directing, to the first recuperation element, incident light that is propagating inside the light guide plate.