To provide an optical diffuser provided with non-compatible materials which are mixed in a liquid phase and are converted to a solid phase and then are formed into a sheet-like optical diffusion material.

(a) A 1st optically transparent material and a 2nd optically transparent material are selected which have different refraction indices, being non-compatible with each other, and having a viscosity ratio between 0.05 and 10 measured at a process temperature; (b) the 1st optically transparent material is dispersed in the 2nd optically transparent material by mechanical mixing like melt-mixing or compounding, etc.; and thus, (c) a film is formed from the mixture. And, this film is cured and is formed into a sheet material.