PURPOSE: To obtain a starch ester having an intermediate DS in one step in an aqueous system by reacting a starch with an organic acid anhydride with a high treatment level in the presence of an aqueous alkali with a high concentration.

CONSTITUTION: A starch is reacted with 35-300 wt.% of an organic acid anhydride (suitably propionic anhydride, acetic anhydride and the like) based on the dry weight of the starch in an aqueous alkaline reaction medium where pH is adjusted to 7-11 by adding an aqueous alkali having a concentration of 10-50 wt.% at 0-40°C to obtain a starch ester having a degree of substitution DS of 0.5-1.8. In this connection, sodium hydroxide is suitable as the alkali. Moreover, the organic acid anhydride is preferably added over four hours.