The 2-cyanoethyl group, which protects the phosphate moiety, generates acrylonitrile as a by-product when deprotected with concentrated ammonia water. Acrylonitrile may cause 2-cyanoethylation through addition reaction to the nucleobase moiety. As a method to avoid the side reaction of acrylonitrile, a weak base reagent such as diethylamine/acetonitrile solution, instead of generally used concentrated ammonia water, can be used to deprotect the 2-cyanoethyl group while maintaining binding with the solid support, followed by cleavage from the solid support and deprotection of the protecting group of the nucleobase moiety.