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.本發明提供一種(甲基)丙烯酸/(甲基)丙烯酸烷酯共聚物及添加該共聚物之化粧料，該(甲基)丙烯酸/(甲基)丙烯酸烷酯共聚物係以(甲基)丙烯酸95.42~97.48質量%、烷基之碳數為18~24之(甲基)丙烯酸烷酯2.43~4.30質量%及具有2個以上乙烯性不飽和基之化合物0.08~0.30質量%作為構成成分。藉由本發明之共聚物，即使於相對較高濃度的電解質存在下，亦可不受溫度及電解質濃度的影響而形成具有高黏度之中和黏稠水溶液，因此可將本發明之共聚物較好地用於化粧料等中。
