[Problem] To provide a 2'-modified ribonucleoside or the like which has a high duplex-forming ability. [Solution] The present invention is generally based on the finding in embodiments that a modified form of a ribonucleoside or the like having an alkoxymethyl protective group can be imparted with a high duplex-forming ability by introducing, as a substituent, a halogen atom into the protective group moiety. As explained before, a modified form of RNA having an alkoxymethyl protective group has a low duplex-forming ability. A modified form of RNA having a halogen-substituted alkoxymethyl protective group, however, exhibits a high duplex-forming ability that is comparable to the duplex-forming ability of a 2'-O-methyl modified nucleic acid.
