Aqueous resin crosslinking agent, aqueous resin crosslinking agent-containing liquid, and aqueous resin composition

A waterborne resin crosslinking agent, comprising a polycarbodiimide compound (A) and a polycarbodiimide compound (B), wherein

• the polycarbodiimide compound (A) has a structure in which isocyanate groups at both terminals are each capped with a hydrophilic organic compound, and at least one of the hydrophilic organic compounds has a molecular weight of 340 or more,
• the polycarbodiimide compound (B) has, as a structural unit, a diisocyanate compound having one cyclohexyl ring or one benzene ring, and has a structure in which isocyanate groups at both terminals are each capped with an organic compound having a molecular weight of 300 or less,
• the polycarbodiimide compound (A) is in an amount of 5 to 90 parts by mass per 100 parts by mass in total of the polycarbodiimide compound (A) and the polycarbodiimide compound (B),
• the polycarbodiimide compound (A) is a product of a decarboxylation condensation reaction of a diisocyanate compound, and
• wherein the degree of polymerization of carbodiimide groups in the polycarbodiimide compound (A) is 5 to 7.