. . . . "A nucleoside analogue of the following formula (I) where B is a pyrimidine or purine nucleic acid base, and X and Y are identical or different, and each represents a hydrogen atom, an alkyl group, an alkenyl group, an alkynyl group, a cycloalkyl group, an aralkyl group, an aryl group, an acyl group, or a silyl group, or an amidite derivative thereof." .