The present invention relates to a simple, economical method for introducing substituent (I) at the 2'-hydroxyl group of the ribose of a ribonucleic acid derivative whose 3'-hydroxyl group and 5'-hydroxyl group are protected with a silicon protecting group, wherein WG.sup.1 represents an electron-withdrawing group: ##STR00001## The invention also relates to a method for producing a ribonucleic acid derivative of formula (3), comprising the reaction of a ribonucleic acid derivative of formula (1) with a monothioacetal compound of formula (2) to produce the ribonucleic acid derivative of formula (3), using iodine as the reagent for halogenating the sulfur atom of the monothioacetal compound of formula (2) in the presence of an acid: ##STR00002## In formulae (1), (2), and (3), Bz represents a nucleobase which may or may not have a protecting group; WG.sup.1 represents an electron-withdrawing group; R.sup.3 represents alkyl or aryl; and A represents a silicon substituent.