A waterborne resin crosslinking agent comprising at least one polycarbodiimide compound selected from the group consisting of a polycarbodiimide compound and a polycarbodiimide compound, a polycarbodiimide compound, and a surfactant, wherein the polycarbodiimide compound has a structure in which an isocyanate group is capped with an end-capping compound represented by the following general formula at both terminals, the polycarbodiimide compound has a structure in which an isocyanate group is capped with the end-capping compound at one terminal and has a structure in which an isocyanate group is capped with an end-capping compound at the other terminal, the polycarbodiimide compound has a structure in which an isocyanate group is capped with an end-capping compound at one terminal and has a structure in which an isocyanate group is end-capped with an end-capping compound at the other terminal: R 1 (OCHR 2 CH 2) n OH wherein R 1 is an alkyl group having 1 to 4 carbon atoms; R 2 is a hydrogen atom or a methyl group; n is an integer of 7 to 30; and the end-capping compound capping one terminal of the polycarbodiimide compound and the end-capping compound capping the other terminal thereof, and the end-capping compound capping one terminal of the polycarbodiimide compound may be the same or different from each other, the end-capping compounds,, and are each independently a compound having one amino group, isocyanate group, epoxy group, or carboxy group, or a compound having one hydroxyl group other than the end-capping compound, a total content of an oxyalkylene group (OCHR 2 CH 2) in the polycarbodiimide compound is 15% by mass or more, a total content of an oxyalkylene group in the polycarbodiimide compounds and based on a total amount of the polycarbodiimide compounds and is 10% by mass or less, and a content of the surfactant is 0.1 to 20 parts by mass based on a total content of the polycarbodiimide compounds and of 100 parts by mass. the polycarbodiimide compound has a structure in which an isocyanate group is capped with an end-capping compound represented by the following general formula at both terminals, the polycarbodiimide compound has a structure in which an isocyanate group is capped with the end-capping compound at one terminal and has a structure in which an isocyanate group is capped with an end-capping compound at the other terminal, the polycarbodiimide compound has a structure in which an isocyanate group is capped with an end-capping compound at one terminal and has a structure in which an isocyanate group is end-capped with an end-capping compound at the other terminal: R 1 (OCHR 2 CH 2) n OH the polycarbodiimide compound has a structure in which an isocyanate group is capped with the end-capping compound at one terminal and has a structure in which an isocyanate group is capped with an end-capping compound at the other terminal, the polycarbodiimide compound has a structure in which an isocyanate group is capped with an end-capping compound at one terminal and has a structure in which an isocyanate group is end-capped with an end-capping compound at the other terminal: R 1 (OCHR 2 CH 2) n OH wherein R 1 is an alkyl group having 1 to 4 carbon atoms; R 2 is a hydrogen atom or a methyl group; n is an integer of 7 to 30; and the end-capping compound capping one terminal of the polycarbodiimide compound and the end-capping compound capping the other terminal thereof, and the end-capping compound capping one terminal of the polycarbodiimide compound may be the same or different from each other, the end-capping compounds,, and are each independently a compound having one amino group, isocyanate group, epoxy group, or carboxy group, or a compound having one hydroxyl group other than the end-capping compound, a total content of an oxyalkylene group (OCHR 2 CH 2) in the polycarbodiimide compound is 15% by mass or more, the end-capping compounds,, and are each independently a compound having one amino group, isocyanate group, epoxy group, or carboxy group, or a compound having one hydroxyl group other than the end-capping compound, a total content of an oxyalkylene group (OCHR 2 CH 2) in the polycarbodiimide compound is 15% by mass or more, a total content of an oxyalkylene group in the polycarbodiimide compounds and based on a total amount of the polycarbodiimide compounds and is 10% by mass or less, and a content of the surfactant is 0.1 to 20 parts by mass based on a total content of the polycarbodiimide compounds and of 100 parts by mass.