A method for purifying an amino acid-N-carboxyanhydride shown by the following general formula: wherein R 1 represents a hydrogen atom, an alkyl group optionally having a substituent, a cycloalkyl group optionally having a substituent, an aryl group optionally having a substituent, a hetero ring optionally having a substituent, or a commonly used protective group for an amino acid selected from a benzyloxycarbonyl group, a t-butoxycarbonyl group, a benzoyl group, and an acetyl group, provided that R 1 does not contain a reactive group of unprotected hydroxy group nor a reactive group of unprotected amino group; X is shown by the following general formula (a) and represents a divalent hydrocarbon group optionally having a substituent, provided that X does not contain a reactive group of unprotected hydroxy group nor a reactive group of unprotected amino group; and R 1 and X are optionally bonded with each other to form a cyclic amino acid structure, wherein R 2 and R 3 each independently represent a hydrogen atom, an alkyl group optionally having a substituent, a cycloalkyl group optionally having a substituent, an aryl group optionally having a substituent, or a hetero ring optionally having a substituent; and "m" is an integer of 1 to 3, the method comprising the steps of: a) dissolving the amino acid-N-carboxyanhydride contaminated with impurities into a solvent which is a good solvent for the amino acid-N-carboxyanhydride 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 suspension obtained in the step a) 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 filtrate obtained by the step b) dropwise to a poor solvent for the amino acid-N-carboxyanhydride to crystallize out the amino acid-N-carboxyanhydride in which the impurities are removed, and wherein the acidic filter aid having ability to trap a basic impurity used in the step b) is selected from magnesium sulfate, calcium sulfate, barium sulfate, copper sulfate, and silica gel having a surface modified with sulfonic acid or carboxylic acid, wherein the good solvent for the amino acid-N-carboxyanhydride used in the step a) is a polar solvent selected from tetrahydrofuran, 1,4-dioxane, ethyl acetate, n-butyl acetate, γ-butyrolactone, acetone, methyl ethyl ketone, methyl isobutyl ketone, dimethyl sulfoxide, N,N-dimethylformamide, and acetonitrile, and wherein the poor solvent for the amino acid-N-carboxyanhydride used in the step c) is a solvent selected from n-hexane, n-heptane, n-octane, n-nonane, n-decane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, diethyl ether, diisopropyl ether, and t-butyl methyl ether. a) dissolving the amino acid-N-carboxyanhydride contaminated with impurities into a solvent which is a good solvent for the amino acid-N-carboxyanhydride 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 suspension obtained in the step a) 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 filtrate obtained by the step b) dropwise to a poor solvent for the amino acid-N-carboxyanhydride to crystallize out the amino acid-N-carboxyanhydride in which the impurities are removed, and