To provide a backlight unit equalizing the light from a light source by a print dot area per given area or a density gradation pattern, in which an amount of light that transmits an optical sheet is improved by a fine roughness provided on the opposite side of a printing surface side.

In the optical sheet that diffuses the light incident from one surface of a transparent base material and emits it from the other surface of the transparent base material, ink having light diffusion properties is printed on the transparent base material with an optional pattern to form a light diffusion part. The pitch of the fine roughness structure formed on the opposite surface of the printing surface of the transparent base material is 30-380 nm, and the aspect ratio is 1.0 or more, the roughness shape being a circular cone or a pyramid, or a pseudo circular cone or a pseudo pyramid.