The oligonucleotide of the present invention includes a sugar-modified nucleoside, the sugar-modified nucleoside has a cross-linked structure between 4-position and 2-position, and the oligonucleotide is capable of binding to the apolipoprotein C-III gene. According to the present invention, an oligonucleotide useful as a therapeutic agent for hyperlipidemia that is excellent in binding affinity to the apolipoprotein C-III gene, stability and safety is provided.