To obtain a material capable of forming a refractive index-modulated image at a low cost without necessitating post-treatment and to provide a process for forming the image.

This material contains a polymethacrylate free from stereoregularity, a vinyl monomer, a thiol compound, a thermal polymerization initiator and a photopolymerization initiator and is produced by compounding the thermal polymerization initiator and the photopolymerization initiator at a molar ratio depending upon the desired refractive index difference. A refractive index-modulated image having refractive index difference between a photo- cured part and a heat-cured part can be formed by coating a substrate with the material, irradiating the material with ultraviolet rays through a mask having a desired pattern to effect the photopolymerization at the exposed part and heating the product to perform the thermal polymerization at the non- exposed part.