A backlight unit comprising: a light guiding plate; a light source module including a plurality of light sources emitting light toward a side surface of the light guiding plate; an under chassis supporting the light guiding plate and the light source module; and a gap maintaining unit maintaining a gap of a constant width between the side surface of the light guiding plate and the plurality of light sources of the light source module when a length of the light guiding plate is changed; wherein the gap maintaining unit comprises a plurality of fixing members to fix the light guiding plate and the light source module to the under chassis, wherein the plurality of fixing members comprises a plurality of studs fixed to a bottom surface of the under chassis, wherein the light guiding plate and the light source module comprises a plurality of through holes (63, 85, 73) into which the plurality of studs is inserted, characterized in that: the light source module is disposed only at one side of the light guiding plate, and the gap maintaining unit comprises a plurality of fixing members to fix the light guiding plate and the light source module to the under chassis, wherein the plurality of fixing members comprises a plurality of studs fixed to a bottom surface of the under chassis. wherein the light guiding plate and the light source module comprises a plurality of through holes (63, 85, 73) into which the plurality of studs is inserted wherein the gap maintaining unit fixing members are only provided towards the end of the light guiding plate adjacent the light source module such that a side surface of the light guiding plate which does not face the light source module is arranged to expand in the lengthwise direction. a light guiding plate; a light source module including a plurality of light sources emitting light toward a side surface of the light guiding plate; an under chassis supporting the light guiding plate and the light source module; and a gap maintaining unit maintaining a gap of a constant width between the side surface of the light guiding plate and the plurality of light sources of the light source module when a length of the light guiding plate is changed; wherein the gap maintaining unit comprises a plurality of fixing members to fix the light guiding plate and the light source module to the under chassis, wherein the plurality of fixing members comprises a plurality of studs fixed to a bottom surface of the under chassis, wherein the light guiding plate and the light source module comprises a plurality of through holes (63, 85, 73) into which the plurality of studs is inserted, characterized in that: the light source module is disposed only at one side of the light guiding plate, and the gap maintaining unit comprises a plurality of fixing members to fix the light guiding plate and the light source module to the under chassis, wherein the plurality of fixing members comprises a plurality of studs fixed to a bottom surface of the under chassis. wherein the light guiding plate and the light source module comprises a plurality of through holes (63, 85, 73) into which the plurality of studs is inserted wherein the gap maintaining unit fixing members are only provided towards the end of the light guiding plate adjacent the light source module such that a side surface of the light guiding plate which does not face the light source module is arranged to expand in the lengthwise direction. the light source module is disposed only at one side of the light guiding plate, and the gap maintaining unit comprises a plurality of fixing members to fix the light guiding plate and the light source module to the under chassis, wherein the plurality of fixing members comprises a plurality of studs fixed to a bottom surface of the under chassis. wherein the light guiding plate and the light source module comprises a plurality of through holes (63, 85, 73) into which the plurality of studs is inserted wherein the gap maintaining unit fixing members are only provided towards the end of the light guiding plate adjacent the light source module such that a side surface of the light guiding plate which does not face the light source module is arranged to expand in the lengthwise direction.