A hydrolysis stabilizer for an ester group-containing resin, which comprises as a main component a carbodiimide having at least two carbodiimide groups in the molecule and resulting from 4,4'-dicyclohexylmethane diisocyanate, isophorone diisocyanate or a mixture of 4,4'-dicyclohexylmethane diisocyanate and isophorone diisocyanate, or from a mixture of tetramethylxylylene diisocyanate with 4,4'-dicyclohexylmethane diisocyanate, isophorone diisocyanate or a mixture of 4,4'-dicyclohexylmethane diisocyanate and isophorone diisocyanate; and a process for the hydrolysis stabilization of an ester group-containing resin, which comprises mixing a polyester type resin with the above hydrolysis stabilizer.