A polysaccharide derivative having a high solubility in an aqueous solvent is produced. The production method of the present invention uses a compound shown by the general formula (1) as a condensing agent and allows a polysaccharide having a carboxyl group to react with an an organic compound having a functional group capable of condensing with the carboxyl group to prepare the polysaccharide derivative: R1 and R2 independently represent a substituent selected among alkyl groups of 1 to 4 carbon atoms and aryl groups of 6 to 8 carbon atoms; Z−represents a counter anion; and E+represents an organic group shown as: R3, R4, and R5 independently represent an organic group having at least one carbon atom directly bound to a quaternary nitrogen atom and any two or all of R3, R4, and R5 may link together to form a cyclic structure.