The present invention provides an optical diffusion device, which can exhibit discontinuously different optical properties only in desired regions without creating any difference in grades and/or breaks, facilitate control of the optical diffusion properties, and prevent the occurrence of the stray light, as well as provides a projection screen and a design member, both using the optical diffusion device. An exemplary optical diffusion device according to this invention includes a base, and a cholesteric liquid crystal polymer layer provided on the base and composed of a plurality of diffusion regions arranged on a plane. A diffusion angle of one of the diffusion regions (e.g., first diffusion regions) of the cholesteric liquid crystal polymer layer is different from a diffusion angle of another of the diffusion regions (e.g., second diffusion regions).