[Problem] To provide a nucleoside that is more practical for RNA pharmaceuticals and other applications and a use therefor. [Solution] A nucleoside derivative indicated by formula (1) or (2) or a salt thereof. (In formula (1), R1 indicates a hydrogen atom, a hydroxyl group, a hydroxyl group having a hydrogen atom substituted by an alkyl group or an alkenyl group, or a protected group. In formula (2), X indicates a halogen atom. In formulas (1) and (2), R2 and R4 can be the same or different and indicate a hydrogen atom, a protecting group for a hydroxyl group, a phosphate group, a protected phosphate group, or P(=0)nR5R6 (n indicates 0 or 1 and R5 and R6 can be the same or different and indicate either a hydrogen atom, a hydroxyl group, a protected hydroxyl group, a thiol group, a protected thiol group, a lower alkoxy group, a cyano lower alkoxy group, an amino group, or a substituted amino group. When n is 1, however, R5 and R6 are never both hydrogen atoms.) R3 indicates NHR7 having a linking group for each (R7 indicating a hydrogen atom, an alkyl group, an alkenyl group, or a protecting group for an amino group), an azide group, an amidino group, or a guanidino group. B indicates a purine-9-yl group, a 2-oxo-pyrimidin-1-yl group, a substituted purine-9-il group, or a substituted 2-oxo-pyrimidin-1-yl group.)
