A process for producing through simple operations a molding die for optical device having an antireflective structure of nano-order microscopic uneven plane on a substratum surface. The molding die for optical device having microscopic uneven plane (antireflective structure die plane) on a surface of substratum is produced by a process comprising forming one or more etching transfer layers on substratum; forming thin film for formation of semispherical microparticles on the etching transfer layers; causing the thin film to undergo aggregation, or decomposition, or nucleation of the material by the use of any of thermal reaction, photoreaction and gas reaction or a combination of these reactions so as to form multiple semispherical islandlike microparticles; and using the multiple islandlike microparticles as a protective mask, carrying out sequential etching of the etching transfer layers and substratum by reactive gas to thereby form a conical pattern on the microscopic surface of the substratum.