An object of the present invention is to provide a copolymer which is capable of forming a viscous aqueous solution having a desired viscosity in the presence of an electrolyte, in particular, a polyvalent electrolyte, a thickening agent comprising the copolymer, and a cosmetic preparation comprising the copolymer. The present invention relates to an acrylic acid ester (A) / compound (B) copolymer, comprising, as constitutional components, an acrylic acid ester (A) and a compound (B), wherein the acrylic acid ester (A) is represented by the general formula [1], and the compound (B) has, in each molecule, a polymerizable unsaturated group and any functional group selected from the group consisting of a carboxyl group, a sulfo group, and a phosphate group, and a weight ratio of the acrylic acid ester (A) and the compound (B), which is (A)/(B), is 9.0/91.0 to 28.5/71.5； a thickening agent comprising the copolymer； and a cosmetic preparation comprising the copolymer； general formula [1]: wherein R 1 represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and m represents an integer of 2 to 4.
