A 2-5A analog represented by the formula (1): 1 wherein m is 0 or 1; n is 0 to 2; R.sup.1 represents an alkoxy group having from 1 to 6 carbon atoms which may be substituted, an unprotected mercapto group, a mercapto group protected by a nucleic acid synthesis protecting group, or an alkylthio group having from 1 to 4 carbon atoms which may be substituted; R.sup.2, R.sup.3, R.sup.4, R.sup.5 and R.sup.6 represent an unprotected hydroxyl group, a hydroxyl group protected by a nucleic acid synthesis protecting group, an alkoxy group having from 1 to 6 carbon atoms which may be substituted, an unprotected mercapto group, a mercapto group protected by a nucleic acid synthesis protecting group, or an alkylthio group having from 1 to 4 carbon atoms which may be substituted; R.sup.7 represents an oxygen atom, or a --O(CH.sub.2CH.sub.2O)q-group, wherein q is 2 to 6; R.sup.8 represents a hydrogen atom, an alkyl group having from 1 to 6 carbon atoms which may be substituted, or a 5'-phosphorylated oligonucleotide analog which has one hydroxyl group removed from the 5'-phosphoric acid group; E.sup.1, E.sup.2, E.sup.3 and E.sup.4 represent a naturally occurring or modified nucleic acid unit, or a pharmacologically acceptable salt thereof.