Disclosed is an organic polymer microparticle which is obtainable from copolymerization of (a) an ethylenically unsaturated amine polymer obtained by reacting an amine polymer having at least two primary amino groups with 0.01 to 0.99 molar equivalents based on the amount of the primary amino group, of an unsaturation introducing monomer having at least one functional group which is reactive with the primary amino group and an ethylenic double bond with (b) 10 to 99000 parts by weight based on the total amount (100 parts by weight) of the amine polymer and the unsaturation introducing monomer, of a matrix forming ethylenically unsaturated monomer, in an aqueous medium. A process for producing the organic polymer microparticle is also disclosed.