A stabilizer against hydrolysis, which can solve conventional problems of a carbodiimide compound compounded in an ester-group-containing resin or a biodegradable plastic as a stabilizer against hydrolysis, such as poor heat resistance and a coloring problem due to yellowing, and can provide enhanced resistance against hydrolysis, and a thermoplastic resin composition of an ester-group-containing resin or a biodegradable plastic containing the same. Specifically, that the stabilizer against hydrolysis includes a carbodiimide composition comprising an aliphatic carbodiimide compound (A) and a phosphorus antioxidant (B), preferably the stabilizer contains a carbodiimide composition wherein a phosphorus antioxidant (B.sup.1) is dispersed and present in the carbodiimide composition by admixing the phosphorus antioxidant (B.sup.1) during synthesis of the aliphatic carbodiimide compound (A), and a thermoplastic resin composition having a superior stability against hydrolysis, characterized in that said stabilizer against hydrolysis is compounded in an easily hydrolysable resin (C).