Abstract: A process for producing a water-base resin dispersion which contains resin particles having a very fine particle diameter as fine as 25 to 100 nm and gives a coating composition excellent in water resistance, alkali resistance and resistance to whitening in hot water, the process being very stable in emulsification and comprising dropwise adding a mixture containing 30 to 100% by weight of said radical-polymerizable ethylenically unsaturated monomer and 20 to 90% by weight of said reactive emulsifier and water to a reactor containing 0 to 70% by weight of said radical-polymerizable ethylenically unsaturated monomer and 10 to 80% by weight of said reactive emulsifier and radical-polymerizing the radical-polymerizable ethylenically unsaturated monomer in the presence of an organic polymerization initiator.