To provide a method of manufacturing a polycarboxylic acid superior in purity and hue, which does not require to go through a decoloring proccess using much decoloring agent and a plurality of crystallization processes, and is not accompanied with by-product of much ammonium salt.

The method of manufacturing the polycarboxylic acid is characterised in that a polycarboxylic acid ester is obtained by reacting a polynitrile compound having at least two nitrile groups in its molecule, water, and a lower alcohol with each other in the presense of a catalyst, and produced ammonia is removed to the outside of the reaction system, the ester is purified by distillation, and then it is hydrolyzed in the presence of an acid catalyst.