A back-light unit including a plurality of substrates, a plurality of light sources respectively disposed on the plurality of substrates and configured to emit light, and N (N≧2) light guide plates respectively disposed adjacent to the plurality of light sources, each light guide plate including a light incidence part having a light incidence surface for receiving light emitted in a first direction from a corresponding light source, and a light emission part for emitting the received incident light in a second direction different than the first direction. Further, at least one of the light guide plates includes light-extracting surface patterns disposed on the light emission part, and the light-extracting surface patterns comprise a first region of light-extracting surface patterns having a higher pattern-density than a second region of light-extracting surface patterns next to the first region.