Provided is an alkyl-modified carboxyl group-containing copolymer with which a neutralized viscous liquid can be prepared which has excellent dispersibility in water, has a viscosity that does not change greatly regardless of addition of an electrolyte, and has high transparency in the presence of an electrolyte. This alkyl-modified carboxyl group-containing copolymer contains: 100 parts by mass of a (meth)acrylic acid; 1.5-4.5 parts by mass of a (meth)acrylic acid alkyl ester, in which an alkyl group has 18-24 carbon atoms; and 0-0.1 parts by mass of a compound having two or more ethylenically unsaturated groups, wherein the copolymer contains 1.5-4.5 parts by mass of a nonionic surfactant.