The present invention provides:a hydrolysis stabilizer for ester group-containing resin, comprising, as the main component, a carbodiimide which is derived from tetramethylxylylene diisocyanate, and which has remaining isocyanate groups in an amount of 3% by weight or less or has the terminal isocyanate groups blocked with a monoisocyanate(s); anda method for hydrolysis stabilization of ester group-containing resin, which comprises mixing an ester group-containing resin with a carbodiimide which is derived from tetramethylxylylene diisocyanate represented by the above formula and which has remaining isocyanate groups in an amount of 3% by weight or less or has the terminal isocyanate groups blocked with a monoisocyanate(s).