PURPOSE: To achieve the efficient removal of a surfactant, a polymerization initiator, an unreacted monomer, a low-molecular byproduct and an inorg. ion, by performing the treatment of an ultrafiltration membrane by using an aqueous polymer emulsion of which the coefficient of variation in its particle size is a specific value or less.

CONSTITUTION: An aqueous polymer emulsion, of which the coefficient of variation in its particle size is about 23% or less, is treated by an ultrafiltration membrane. This coefficient of variation is calculated on the basis of formula σN-1/d×100 after the particle diameter (d) of the aqueous polymer emulsion is practically measured with respect to N-numbers of particles from the photograph of said emulsion by an electronic microscope and the average value (d) and reference deviation σN-1 thereof are calculated. In this case, a vinyl polymer emulsion, which is obtained by copolymerizing two or more of vinyl monomers, is used. The fractionation M.W. of the ultrafiltration membrane is pref. 500W500,000.