The present invention provides a process for producing an N-acyl acidic amino acid or a salt thereof, which comprises an amidation reaction between a fatty acid acyl chloride and an amino acid under alkaline conditions. The feature of this method is that in the amidation reaction, water is used as a solvent, acidic amino acid or a salt thereof as a main raw material, and a small amount of neutral amino acid or a salt as an auxiliary material. Is added dropwise to an aqueous solution of an acidic amino acid or a salt thereof; an alkali is added to adjust the pH value of the reaction solution; after adding a predetermined amount of a fatty acid acyl chloride, an aqueous solution of a neutral amino acid or a salt thereof is added; Subsequently, the fatty acid acyl chloride is added dropwise to the end and stirred to maintain the reaction. In this method, by reacting the mixed amino acid and fatty acid acyl chloride in an aqueous phase system, the conversion ratio of amino acid and acyl chloride can be greatly increased, and the amount of residual amino acid can be greatly reduced. Since the product can be used as a surfactant as it is with a simple treatment, the cost can be greatly reduced.本発明は、アルカリ条件下での脂肪酸塩化アシルとアミノ酸とのアミド化反応を含む、N-アシル酸性アミノ酸又はその塩の製造方法を提供する。本方法の特徴は、アミド化反応において、水を溶剤とし、酸性アミノ酸又はその塩を主要原料とし、少量の中性アミノ酸又は塩を補助材料とし、以下の工程:攪拌条件で、まず脂肪酸塩化アシルを酸性アミノ酸又はその塩の水溶液中に滴加し;反応液のpH値を調節するためにアルカリを加え;脂肪酸塩化アシルを所定量滴加した後、中性アミノ酸又はその塩の水溶液を加え、続いて脂肪酸塩化アシルを最後まで滴加し、撹拌して反応を維持することを含む。本方法は、混合されたアミノ酸と脂肪酸塩化アシルとを水相系で反応させることで、アミノ酸と塩化アシルの転化率を大幅に高め、残余アミノ酸量を大幅に減らすことができる。生成物は簡単な処置でそのまま界面活性剤として使用できるため、コストを大幅に減らすことができる。