A 2-5A analog represented by the formula (1): wherein m is 0 or 1; n is 0 to 2; R1 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; R2, R3, R4, R5 and R6 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; R7 represents an oxygen atom, or a —O(CH2CH2O)q- group, wherein q is 2 to 6; R8 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; E1, E2, E3 and E4 represent a naturally occurring or modified nucleic acid unit, or a pharmacologically acceptable salt thereof.