An amidite reagent (phosphoramidite) is a unit used for a coupling reaction in nucleic acid synthesis. The 3' position of a nucleoside or deoxynucleoside has a phosphate ester structure, and a cyanoethoxy group and a dialkylamino group are respectively bonded to the phosphorus atom. When an amidite reagent and activator act on a substrate, an amino group moiety detaches and a coupling reaction with the 5' position of the substrate proceeds. Chemical modifications are introduced to the base, phosphate, or carbohydrate moiety of nucleic acid therapeutics in order to improve functionality by acquiring resistance to nucleic acid degrading enzymes (nucleases) or improving the binding affinity to target nucleic acids. Fujifilm Wako can supply cross-linked artificial nucleic acids (LNA) that are resistant to nucleases and are expected to be applied to nucleic acid therapeutics.