A display device having a prism sheet concentrating a spreading angle of light from a light source upon a certain range and a reflective polarizer facing the prism sheet is used conventionally.  When the light passes through the prism sheet, the reflective polarizer transmits, for example, the p-polarized component of the light to a display panel (such as a liquid crystal panel) side and reflects the s-polarized component of the light onto a light source side. 
The s-polarized component reflected by the reflective polarizer is again reflected by, for example, an optical element such as a reflective plate provided on the light source side, and reaches back the reflective polarizer.  Such a reflection process is repeated, and the s-polarized component converts into the p-polarized component by-and-by and passes through the reflective polarizer.  When the reflective polarizer is used, the utilization efficiency of the light from the light source increases and thus, the brightness of the display device increases. 
In a display device with such a structure, interference fringes may occur between the prism sheet and the reflective polarizer.  The interference fringe is one of the causes of display quality deterioration in the display device.  Therefore, reduction or prevention of the occurrence of interference fringes is demanded.