PURPOSE:To industrially obtain the titled compound useful as a mucilage dissolvent, etc., without containing inorganic salts in high yield, by neutralizing an acetylation reaction solution of an amino acid and removing metallic ions with an electrodialytic apparatus using cation exchange membranes as diaphragms. CONSTITUTION:An acetylation reaction solution 3 of an amino acid is circulated, neutralized and subjected to removal of metallic ions with an electrodialytic apparatus using carbon exchange membranes 4, e.g. p-styrenesulfonic acid- divinylbenzene copolymer, etc., to afford the aimed highly water-soluble N- acetylamino acids. The electrodialytic apparatus is a filter press type apparatus prepared by sandwiching diaphragms with compartment frames and providing the anode plate and the cathode plate at both ends. C, Pt, lead plate, etc., are used as the anode plate and Ag, iron, etc., are used as the cathode plate. A pole solution 1 (aqueous solution of sulfuric acid) is circulated on the side of both electrode plates and a 1-2N aqueous solution 2 of a mineral acid, e.g. HCl, H2SO4, etc., and reaction solution 3 are circulated from the anode side.