The present invention is a thickener consisting of a microgel obtained by radical polymerization of water soluble ethylene-type unsaturated monomers dissolved in the dispersion phase in a composition having an organic solvent or an oil component as the dispersion medium and water as the dispersion phase, as well as a cosmetic containing said thickener. Also, the present invention is a method of manufacturing a water-swelling polymer in a radical polymerization system consisting of a composition having an organic solvent or oil component as the dispersion medium and an aqueous solution of a water soluble ethylene type unsaturated monomer as the dispersion phase wherein said system contains a surfactant chosen in such a way that the aforementioned composition forms a single phase W/O microemulsion or fine W/O emulsion at a thermal radical polymerization temperature and said thermal radical polymerization is carried out at a temperature equal to or higher than the phase transition temperature of the radical polymerization system, yet not higher than the phase transition temperature by 20° C. or more.