To provide a polycarbodiimide copolymer and a modified polycarbodiimide copolymer each suitable for a curing agent that can allow a cured product of a carboxy group-containing resin composition to be improved in flexibility without any loss of water resistance, as well as a curing agent of a carboxy group-containing resin composition, and a carboxy group-containing resin composition. The present invention enables a cured product to be enhanced in water resistance due to introduction of a predetermined soft segment into a polycarbodiimide molecule, in use for a curing agent, and also enables a carboxy group-containing resin composition to be enhanced in storage stability due to modification with an aliphatic amine or an aromatic heterocyclic amine.