A backlight unit includes: a light guide plate of an entirely flat structure; a light emitting element disposed on at least one side of the light guide plate; a first reflection part disposed under the light guide plate and having one side thereof extended to a region where the light emitting element is positioned; a second reflection part disposed on the light emitting element; and a housing for accommodating the light guide plate, the light emitting element, and the first and second reflection parts, wherein the housing has a second side which is vertically extended upwards with respect to a first side which defines a lateral cross section of the housing in which the light emitting element is accommodated as the lower side, and a third side which is vertically extended inwards with respect to the second side.