Disclosed is a water-soluble copolymer having an alkyl-modified carboxyl group, which can form a viscous nutralizing solution having a satisfactorily high viscosity, an extremely high transmittance, no greasy feeling and a fresh texture even in the presence of a relatively high concentration of an electrolyte. Also disclosed is a thickening agent comprising the water-soluble copolymer. The water-soluble copolymer can be produced by polymerization of 100 parts by weight of (meth)acrylic acid, 5 to 10 parts by weight of lauryl (meth)acrylate and 0 to 0.1 part by weight of a compound having two or more ethylenically unsaturated groups. The water-soluble copolymer shows a viscosity of 1000 mPa·s or lower and a transmittance of 95% or higher when prepared into a viscous neutralizing solution containing 1% by weight of the water-soluble copolymer, and shows a maximum viscosity of 10000 to 30000 mPa·s and a transmittance of 95% or higher when 1 to 7 parts by weight of sodium chloride is added to 100 parts by weight of the viscous neutralizing solution.