The object of the present invention is to provide novel oligonucleotide analogues, which exhibit antisense or antigene activity having excellent stability, or exhibit excellent activity as a detection agent (probe) for a specific gene or as a primer for starting amplification, and to novel nucleoside analogues which are intermediates for their production. ÄSolutionÜ A compound of the formula (1): <；CHEM>； Äwherein R<；1>； and R<；2>； are the same or different and represent a hydrogen atom, a hydroxyl protecting group, a phosphoric acid group, or -P(R<；3>；)R<；4>； Äwherein R<；3>； and R<；4>； are the same or different and represent a hydroxyl group, an amino group, an alkoxy group having from 1 to 4 carbon atoms, a cyanoalkoxy group having from 1 to 5 carbon atoms or an amino group substituted by an alkyl group having from 1 to 4 carbon atomsÜ； A represents an alkylene group having from 1 to 4 carbon atoms； and B represents a purin-9-yl group, a 2-oxo-pyrimidin-1-yl group or a substituted purin-9-yl group or a substituted 2-oxo-pyrimidin-1-yl group having a substituent selected from the following alpha groupÜ； or the salt thereof； ( alpha group>； a hydroxyl group which may be protected, an alkoxy group having from 1 to 4 carbon atoms, a mercapto group which may be protected, an alkylthio group having from 1 to 4 carbon atoms, an alkoxy group having from 1 to 4 carbon atoms, an amino group which may be protected, a mono- or di-alkylamino group which may be substituted by an alkyl group having from 1 to 4 carbon atoms, an alkyl group having from 1 to 4 carbon atoms and a halogen atom. A compound of the formula (I), and the pharmacologically acceptable salts or derivatives thereof.
