A backlight unit capable of preventing an initial driving malfunction is disclosed. The backlight unit includes a bottom cover opened upwardly, a reflective sheet disposed in the inner surface of the bottom cover; a plurality of lamps arranged at a fixed interval on the reflective sheet; an auxiliary light source disposed at the outer side surface of the bottom cover; and a light guide member, opposite to the emitting surface of the auxiliary light source, configured to guide light emitted from the auxiliary light source to the lamps through first and second penetration holes formed on one of the side walls of the bottom cover and the reflective sheet.