Abstract: An emulsion polymerization is carried out using one or more polymerizable unsaturated monomers X having at least one carbon-carbon double bond in a molecule, one or more reactive emulsifiers Y having at least one polymerizable group in a molecule, and one or more polyfunctional monomers Z1 having two or more polymerizable unsaturated groups in a molecule and having a molecular weight per functional group of less than 150, or one or more hydrophilic monomers Z2 having one polymerizable unsaturated group and a hydrophilic group in a molecule and having an average molecular weight of less than 250, with a proportion of the total mass of the polyfunctional monomer Z1 or hydrophilic monomer Z2 to the total mass of the reactive emulsifier Y being in the range of Z1/Y=1/3 to 1/30 or Z2/Y=1/1 to 1/30.