Provided is a novel molecule for an antisense by which the expression of a specific gene can be efficiently regulated. As formulae (I) and (II) show, the novel artificial nucleoside has an amide bond introduced into a bridged structure of 2',4'-BNA/LNA. An oligonucleotide containing this 2',4'-bridged artificial nucleotide has a binding affinity for a single-stranded RNA comparable to that of publicly known 2',4'-BNA/LNA, and a resistance to nuclease stronger than that of LNA. Since the binding affinity of the aforesaid oligonucleotide to a single-stranded RNA is much stronger than that of S-oligo, in particular, it is expected that the oligonucleotide is applicable to nucleic acid drugs.
