PURPOSE: To provide the copolymer and its water-soluble salt useful as a thickener having an excellent rheology-modifying ability by emulsion-copolymerizing a radically polymerizable ethylenically unsaturated monomer with a carboxyl- containing monomer.

CONSTITUTION: There is provided the objective copolymer based on a radically copolymerizable ethylenically unsaturated monomer (A), the macromolecule of which contain 21-99 wt.% (desirably, 25-70 wt.%) carboxyl-containing monomer units and are prepared by emulsion copolymerization initiated by free radicals in the presence of 0.1-24 wt.% polyvinyl alcohol(PVA) or polyvinylpyrrolidone(PVP) and having a solids content of, desirably 10-50 wt.% (particularly desirably, 20-40 wt.%).