To prevent diffracted light from being colored into rainbow colors, prevent a screen from being colored, prevent the occurrence of moire fringes, prevent the diffusion of an image from being increased, and prevent a light source such as the sun and the background from being displayed on the screen by modulating the grating period of a diffraction grating pattern with a smooth pattern having the period larger than the grating period.

The diffraction grating pattern 51 of the fundamental period Λ0 is modulated by a smooth function (pattern) 52 having a period P longer than the fundamental period Λ0 to obtain this diffraction grating 41, the grating period is changed in a range of 15-25 μm, and the modulation period P is set at 80 μm. The modulation period P is 1/2 or below of the pixel period, and the occurrence of moire fringes on a screen is prevented. The diffraction angle is not fixed, and the diffracted light is rarely colored into rainbow colors. When light is diffracted by the diffraction grating 41, the light with the same wavelength is dispersed in response to the modulation degree of the grating period, and the light with different wavelengths is hardly overlapped into rainbow colors.