A carbodiimide-based aqueous resin crosslinking agent excellent in the storage stability in the coexistence of an aqueous resin, comprising a polycarbodiimide (A) having substituents derived from hydrophilic organic compounds at both terminals thereof, at least one of the substituents being a substituent derived from a hydrophilic organic compound having a molecular weight of 340 or more, and a polycarbodiimide (B) having substituents derived from organic compounds having a molecular weight of 300 or less at both terminals thereof, wherein the mass ratio [(B)/(A)] of the polycarbodiimide (B) to the polycarbodiimide (A) is from 20/80 to 95/5.