To provide an optical sheet capable of diffusing light after passing through each Fresnel lens surface 1a to some extent in each region R and preventing the generation of a dark line due to the reduction of front side brightness in the vicinity of the boundary of the region R, and to provide a backlight unit using the optical sheet.

In a planoconvex linear Fresnel lens type optical sheet 1, each Fresnel lens surfaces 1a divided like a planoconvex linear Fresnel lens is divided into a plurality of regions R in each plurality of adjacent Fresnel lens surfaces 1a. The Fresnel lens surface 1a of the central part R1of the region R is inclined on the right and left end sides in a reverse rotational direction and at an angle being larger as it goes to the end sides and the Fresnel lens surfaces 1a of the end potions R2of the region R are inclined at an angle being larger as it goes to the side of the central part R1.

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