A binder for a nonaqueous electrolyte secondary battery electrode, containing a crosslinked polymer or salt thereof having a carboxyl group and a use thereof, and a method of manufacturing the polymer or salt. The polymer has a structural unit derived from an ethylenically unsaturated carboxylic acid monomer in an amount of 50 to 99 mass % of total structural units and a structural unit derived from a nonionic ethylenically unsaturated monomer in an amount of 1 to 50 mass % of the total structural units, the monomer is a compound having a substituent with a carbon atom number of 6 or more, and a particle diameter of the crosslinked polymer is 0.1 to 7.0 μm in a volume-based median diameter when the crosslinked polymer is neutralized to a neutralization degree of 80 to 100 mol %, subjected to water swelling in water, and then dispersed in a 1 mass % NaCl aqueous solution.