To provide a method for producing a conjugate of polynucleotide and polysaccharide.

The method comprises (a) a step of preparing an aldonic acid of polysaccharide or a derivative thereof; (b) a step of forming an aldonic acid ester, preferably activated one, by reacting the aldonic acid with an alcohol derivative, preferably the alcohol carbonate derivative, in an anhydrous aprotic polar solvent; and (c) a step of reacting the resultant aldonic ester with a polynucleotide (wherein the polynucleotide contains functional amino groups).