The present invention provides a novel nucleoside derivative or a salt thereof, a polynucleotide synthesis reagent, a method for producing a polynucleotide, a polynucleotide, and a method for producing a binding nucleic acid molecule. The nucleoside derivative or a salt thereof of the present invention is represented by the following chemical formula (1): where in the chemical formula (1), Su is an atomic group having a sugar skeleton at a nucleoside residue or an atomic group having a sugar phosphate skeleton at a nucleotide residue, and may or may not have a protecting group, L1 and L2 are each independently a straight-chain or branched, saturated or unsaturated hydrocarbon group having 2 to 10 carbon atoms, X1 and X2 are each independently an imino group (—NR1—), an ether group (—O—), or a thioether group (—S—), and the R1 is a hydrogen atom or a straight-chain or branched, saturated or unsaturated hydrocarbon group having 2 to 10 carbon atoms.