The present invention is to provide a copolymer capable of forming a viscous aqueous solution having a desired viscosity in the presence of an electrolyte, particularly a polyelectrolyte； a thickener comprising the copolymer； and cosmetics formulated with the copolymer.The present invention is related to copolymer of acrylic ester (A)/compound (B), thickener comprising the copolymer, and cosmetics formulated with the copolymer； wherein the copolymer of acrylic ester (A)/compound (B) is constituted of an acrylic acid ester (A) represented by the general formula (1), and (B) a compound having a polymerizable unsaturated group and any functional group selected from the group consisting of a carboxyl group, a sulfo group and a phosphoric acid group in the molecule. The ratio of said acrylic acid ester (A) to compound (B) (by weight) is in the range of (A)/(B)=9.0/91.0 to 28.5/71.5.(Wherein, R1 represents a hydrogen atom or an alkyl group having 1 to 6 carbon atoms, and m represents an integer of 2 to 4.)本發明之目的在於提供一種在電解質的存在下，尤其在多價電解質的存在下可形成具有所求黏度之黏稠水溶液之共聚物，含有該共聚物之增黏劑，及調配有該共聚物之化妝料。本發明係關於丙烯酸酯(A)/化合物(B)共聚物、含有該共聚物之增黏劑，以及調配有該共聚物之化妝料，其中該丙烯酸酯(A)/化合物(B)共聚物係以(A)通式[1]所示丙烯酸酯、以及(B)分子內具有聚合性不飽和基及由羧基、磺基及磷酸基所成群組中選擇的任一官能基之化合物作為構成成分，且上述丙烯酸酯(A)與上述化合物(B)之比例(重量比)為(A)/(B)=9.0/91.0~28.5/71.5。通式[1]：(式中，R1表示氫原子或碳數1~6之烷基，m表示2~4之整數。)
