The present invention is a method for purifying an NCA, including the steps of: a) dissolving an NCA contaminated with impurities into a solvent which is a good solvent and is not a chlorinated solvent followed by stirring to precipitate an undissolved impurity to afford a suspension, b) adding an acidic filter aid having ability to trap a basic impurity to the obtained suspension followed by filtration and/or forming a fixed bed of the acidic filter aid having ability to trap a basic impurity followed by filtering the suspension to bring the suspension to be in contact with the acidic filter aid having ability to trap a basic impurity, and c) adding the obtained filtrate dropwise to a poor solvent for NCA to crystallize out the NCA in which the impurities are removed. This makes it possible to purify a low-purity NCA conveniently to afford a high-purity NCA.