A backlight unit comprising: a housing (135, 300) including a plurality of first fixing parts protruding from the housing (135, 300) and a plurality of second fixing parts protruding from the housing (135, 300); a plurality of light source devices; and at least one optical sheet disposed above the plurality of light source devices, the at least one optical sheet including: - a plurality of upper holes at an upper side of the at least one optical sheet and coupled to the plurality of first fixing parts, - a plurality of lower holes at a lower side of the at least one optical sheet and coupled to the plurality of second fixing parts, and a lateral side forming a first corner with the upper side and a second corner with the lower side of the at least one optical sheet, wherein the plurality of upper holes includes at least a first hole having a first distance to the first corner along the upper side, and a second hole having a second distance to the first corner along the upper side, wherein the first distance is smaller than the second distance, wherein the plurality of lower holes includes at least a first hole having a third distance to the second corner along the lower side, and a second hole having a fourth distance to the second corner along the lower side, wherein the third distance is smaller than the fourth distance, and wherein a size of the second hole of the plurality of upper holes is smaller than a size of the first hole of the plurality of upper holes, characterized in that the first distance differs from the third distance, and the second distance differs from the fourth distance. a housing (135, 300) including a plurality of first fixing parts protruding from the housing (135, 300) and a plurality of second fixing parts protruding from the housing (135, 300); a plurality of light source devices; and at least one optical sheet disposed above the plurality of light source devices, the at least one optical sheet including: - a plurality of upper holes at an upper side of the at least one optical sheet and coupled to the plurality of first fixing parts, - a plurality of lower holes at a lower side of the at least one optical sheet and coupled to the plurality of second fixing parts, and a lateral side forming a first corner with the upper side and a second corner with the lower side of the at least one optical sheet, - a plurality of upper holes at an upper side of the at least one optical sheet and coupled to the plurality of first fixing parts, - a plurality of lower holes at a lower side of the at least one optical sheet and coupled to the plurality of second fixing parts, and wherein the plurality of upper holes includes at least a first hole having a first distance to the first corner along the upper side, and a second hole having a second distance to the first corner along the upper side, wherein the first distance is smaller than the second distance, wherein the plurality of lower holes includes at least a first hole having a third distance to the second corner along the lower side, and a second hole having a fourth distance to the second corner along the lower side, wherein the third distance is smaller than the fourth distance, and wherein a size of the second hole of the plurality of upper holes is smaller than a size of the first hole of the plurality of upper holes, characterized in that the first distance differs from the third distance, and the second distance differs from the fourth distance.