To produce an aqueous resin dispersion, having a small particle diameter and cross-linking properties, good in stability at the time of polymerization or preservation, and excellent in film-forming properties and water and solvent resistances of a coating film, by radically polymerizing a radically polymerizable unsaturated monomer with a radically polymerizable reactive emulsifying agent with an organic polymerization initiator.

A mixture containing 30-10wt.% of (A) 100 pts.wt. of a radically polymerizable unsaturated monomer (e.g. methyl acrylate) containing preferably 1-50wt.% of an acetoacetyl group-containing unsaturated monomer (e.g. acetoacetoxyethyl acrylate), 20-90wt.% of (B) 0.1-50 pts.wt. of a reactive emulsifying agent having a radically polymerizable unsaturated double bond in the molecule (e.g. a sulfosuccinic acid ester-based emulsifying agent) and water is dropped into a reactional vessel containing water, 0-70wt.% of the component A and 10-80wt.% of the component B when carrying out the radical polymerization of the components A and B with (C) an organic polymerization initiator (e.g. tert-butyl perbenzoate). Thereby, a dispersion having 10-100nm average particle diameter is obtained. A cross-linking agent can be compounded therewith.