Oligonucleotide analogues which have anti-sense or anti-gene activity, as well as in vivo stability, or pharmaceutically acceptable salts thereof. The oligonucleotide analogues have one or more structural units represented by the following formula (1a): provided that when the oligonucleotide has two or more structural units of formula (1a), each B is the same or different, wherein B represents a purin-9-yl group or a 2-oxo-1,2-dihydropyrimidin-1-yl group.