A (meth)acrylic acid/alkyl (meth)acrylate ester copolymer containing constituents of 95.42 to 97.48% by mass of (meth)acrylic acid, 2.43 to 4.30% by mass of an alkyl (meth)acrylate ester of which alkyl group has 18 to 24 carbon atoms, and 0.08 to 0.30% by mass of a compound having two or more ethylenically unsaturated groups； and cosmetics blended with the copolymer. According to the copolymer of the present invention, a neutral viscous aqueous solution having a high viscosity can be formed even in the presence of an electrolyte in a relatively high concentration, without being affected by its temperature and electrolytic concentration； therefore, the copolymer of the present invention can be suitably used in cosmetics and the like.
