An antisense oligonucleotide derivative against HCV is provided which contains one or more nucleotide analogue units having a modified sugar portion and represented by the following general formula where B denotes a pyrimidine or purine nucleic acid base or an analogue thereof. The derivative of the present invention is an antisense against hepatitis C virus (HCV) gene, binds to HCV-RNA with high affinity within cells, can control and inhibit the expression of its gene with high efficiency, and shows high resistance to nucleases. The BNA antisense oligonucleotide of the present invention is also effective in an antisense method targeting HCV, no matter what secondary structures, such as loops or stems, in a target RNA nucleic acid are.