A backlight unit for use in an LCD unit includes an optical guide plate, a tubular lamp extending along the lateral side of the optical guide plate, a reflection member including a reflector encircling the tubular lamp at three sides thereof and a reflection sheet disposed at the rear side of the optical guide plate, a chassis receiving therein the optical guide plate and the reflection member, and a lamp holder disposed at the corner portion of the chassis. The reflector has an extension at the corner portion, the extension being sandwiched between the chassis and the lamp holder to restrict the lateral movement of the reflector.