A backlight unit comprising a plurality of small light sources, the backlight unit configured to be built into a liquid crystal display device such that the plurality of small light sources are arranged in a dispersed manner at a side of the liquid crystal display device opposite to a display panel, the backlight unit configured to guide light emitted from the plurality of small light sources towards the display panel, wherein: the backlight unit further includes a prism sheet and a plurality of optical sheets overlapped with one another and configured to be arranged between the display panel and the plurality of small light sources, the plurality of optical sheets being interposed between the plurality of small light sources and the prism sheet; at least one surface of each of three or more optical sheets of the plurality of optical sheets has unevenness in which recesses each having an inverted pyramid shape are two-dimensionally arrayed, the plurality of optical sheets comprise 0 part by weight or more and 2 parts by weight or less of a diffusing agent with respect to 100 parts by weight of a matrix resin constituting each of the optical sheets, a distance between the plurality of small light sources and the plurality of optical sheets is 2 mm or less, when at least one scope of 0.5 mm × 0.5 mm or more of the surface of the plurality of optical sheets having the unevenness is measured, height data of each of a plurality of pixels of an image obtained is determined, an approximated flat surface is calculated, from the height data of each pixel, for a minute region of 100 μm 2 or less including a plurality of pixels, and a calculation is repeatedly performed to obtain an inclination angle between: (i) a flat surface with a height of 0 being parallel to a main plane of extension of the plurality of optical sheets and: (ii) the approximated flat surface, while two-dimensionally shifting the minute region at equal intervals along the flat surface by using at least one of the pixels as a unit to obtain a plurality of minute regions, a total area of some of the minute regions each having the inclination angle of 30° or more, accounts for 30% or more of a total area of all the minute regions, for which the calculation is made. the backlight unit further includes a prism sheet and a plurality of optical sheets overlapped with one another and configured to be arranged between the display panel and the plurality of small light sources, the plurality of optical sheets being interposed between the plurality of small light sources and the prism sheet; at least one surface of each of three or more optical sheets of the plurality of optical sheets has unevenness in which recesses each having an inverted pyramid shape are two-dimensionally arrayed, the plurality of optical sheets comprise 0 part by weight or more and 2 parts by weight or less of a diffusing agent with respect to 100 parts by weight of a matrix resin constituting each of the optical sheets, a distance between the plurality of small light sources and the plurality of optical sheets is 2 mm or less, when at least one scope of 0.5 mm × 0.5 mm or more of the surface of the plurality of optical sheets having the unevenness is measured, height data of each of a plurality of pixels of an image obtained is determined, an approximated flat surface is calculated, from the height data of each pixel, for a minute region of 100 μm 2 or less including a plurality of pixels, and a calculation is repeatedly performed to obtain an inclination angle between: (i) a flat surface with a height of 0 being parallel to a main plane of extension of the plurality of optical sheets and: (ii) the approximated flat surface, while two-dimensionally shifting the minute region at equal intervals along the flat surface by using at least one of the pixels as a unit to obtain a plurality of minute regions, a total area of some of the minute regions each having the inclination angle of 30° or more, accounts for 30% or more of a total area of all the minute regions, for which the calculation is made.