A 2′-modified ribonucleoside 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. A modified form of RNA having a halogen-substituted alkoxymethyl protective group exhibits a high duplex-forming ability that is comparable to the duplex-forming ability of a 2′-O-methyl modified nucleic acid.