To obtain stabilized monoaminocarboxylic acids without being discolored while being stored, transported, and used, and useful as a biodegradable chelating agent by specifying the whole amount of a cyanogen ion, hydrogen cyanide and its salt.

In this stabilized monoaminocarboxylic acids of the formula [R1 is H or a 1-10C (substituted) hydrocarbon; R2 and R3 are each H or a 1-8C (substituted) hydrocarbon; R4 is H, OH, CO2M, SO3M (M is H, an alkali metal, etc.)] (preferably aspartic acid-N-monoacetate, etc.) includes the whole amount of a cyanogen ion, hydrogen cyanide and its salt are regulated to ≤100ppm, the whole amount of an ammonium ion, ammonia and its salt is regulated to ≤1% and the whole amount of formaldehyde is regulated to ≤1% based on the carboxylic acids. The carboxylic acids are obtained, e.g. by performing an addition reaction of a raw amino acid with prussic acid and formaldehyde and further hydrating the addition reaction products under an alkaline condition.