FIG.  12 is a sectional view showing a conventional liquid crystal display device 1.  The liquid crystal display device, of the direct-lit type, is composed of an illuminating device 2 and a liquid crystal panel 3; the illuminating device 2 is disposed behind the liquid crystal panel 3.  In the illuminating device, a plurality of linear light sources 4 are arrayed in parallel, and between the liquid crystal panel 3 and the illuminating device 2, there are disposed an optical sheet 7, which includes a prism sheet 6, and a diffusion plate 8.  In such a liquid crystal display device 1, the light emitted from the linear light sources 4 passes through the diffusion plate 8 and the optical sheet 7, and reaches the liquid crystal panel 3. 
The optical sheet 7 is composed of various sheets such as a diffusion sheet 5, a prism sheet 6, a lens sheet, and a polarizing film, and is so disposed as to cover the main surface of the diffusion plate 8.  FIG.  13 is an exploded perspective view showing, among the optical sheet, in which those sheets are laid together, parts of the diffusion sheet 5 and the prism sheet 6.  As shown in FIG.  13, the prism sheet 6 is used, with a view to enhancing the front-face brightness of the liquid crystal display panel 3, to condense the light from the linear light sources 4 toward the liquid crystal panel 3, and has a prism surface formed on its main surface facing the liquid crystal panel.  This fine prism surface is composed of first prism rows 6a formed with a predetermined pitch p and having a triangular cross section. 
FIG.  14 is a sectional view showing the state of the prism sheet 6 and the diffusion sheet 5 in FIG.  13 laid on one another; in a case where the optical sheet is composed of the plurality of sheets laid together as shown in FIG.  14, the peaks of the first prism rows 6a on the surface of the prism sheet 6 make contact with the main surface of the abutting diffusion sheet 5.  With this structure, when an external force acts on the optical sheet, the peaks of the first prism rows 6a may rub against the main surface of the diffusion sheet 5 and become scratched.  If these scratches are visible from the viewing direction of the display device, they constitute a problem in the quality of the liquid crystal display device. 
Thus, to solve this problem, conventionally, methods have been proposed (see Patent Documents 1 and 2) in which the peaks of the prism rows are not made sharp but are formed to have gently curved surfaces.     Patent Document 1: JP-A-2004-110025 Publication    Patent Document 2: JP-A-2005-317435 Publication