Bicyclonucleoside analogues which exhibit anti-AIDS activity and intermediates to produce oligonucleotide analogues which have anti-sense or anti-gene activity as well as in vivo stability. Compounds of the following formula (1) or their pharmaceutically acceptable salts. ##STR00001## wherein R.sup.1 represents a hydrogen atom or a protecting group for a hydroxy group, R.sup.2 represents an azido group or an optionally protected amino group or the like, B represents a purin-9-yl or a 2-oxo-1,2-dihydropyrimidin-1-yl group, which are optionally substituted with substituents selected from the group consisting of a halogen atom, an alkyl group having 1 6 carbon atoms, a hydroxy group, a mercapto group and an amino group.