Provided is a backlight unit with an extending portion that extends from one of a plurality of sheets included in an optical sheet. The optical sheet is on a light guide plate, and a light emitting diode chip is disposed at a side of the light guide plate. A portion of a flexible printed circuit board, to which the light emitting diode chip is mounted, overlaps a portion of the light guide plate. The extending portion overlaps the portion of the flexible printed circuit board. Thus, the occurrence of lines of light due to light progressing between the light guide plate and the flexible printed circuit board can be prevented, thereby obtaining uniform brightness and improving image quality.