A nucleoside derivative represented below, or a salt thereof.




(In (1), R1 represents a hydrogen atom, a hydroxyl group or a protected group, and in (2), X represent a halogen atom. In (1) and (2), R2 and R4 each represent a hydrogen atom, a hydroxyl protecting, phosphate, or protected phosphate group, or —P(═O)nR5R6 (n is 0 or 1, R5 and R6 each representing a hydrogen atom, hydroxyl, protected hydroxyl, mercapto, protected mercapto, lower alkoxy, cyano lower alkoxy, amino or substituted amino group, when n is 1, R5 and R6 are not both hydrogen atoms), R3 represents NHR7 (R7 represents a hydrogen atom, alkyl, alkenyl or protecting group for an amino group), an azide, amidino or guanidino group, each having a linking group (when R7 is hydrogen atom, the linking group is an alkylene group), and B represents any of a purine-9-yl, 2-oxo-pyrimidin-1-yl, substituted purine-9-yl or substituted 2-oxo-pyrimidin-1-yl group).