POLYESTER RESIN MODIFYING AGENT, METHOD FOR PRODUCING SAME, AND POLYESTER RESIN COMPOSITION

The present invention provides: a polyester resin modifying agent which contains a polycarbodiimide compound and is capable of improving the hydrolysis resistance of a polyester resin, and which is produced using only a small amount of a carbodiimidization catalyst and is suppressed in the degree of yellowing; a method for producing this polyester resin modifying agent; and a polyester resin composition which contains this modifying agent. A polyester resin modifying agent which contains a polycarbodiimide compound that is obtained by reacting an aliphatic diisocyanate, an isocyanate terminal blocking agent and a carbodiimidization catalyst, and which is configured such that: the content of the carbodiimidization catalyst is within a predetermined range; and the degree of yellowing (YI) thereof is less than 20. A method for producing this polyester resin modifying agent; and a polyester resin composition.