[Object] An object of the present invention is to provide a surface-treating agent excellent in antithrombogenicity, and accordingly, excellent in biocompatibility, and having high hydrophilicity as compared with conventional medical materials. Further, an object of the present invention is to provide a production method excellent in performance of removing unreacted monomers and further improving a yield as compared with conventional production methods. [Constitution] The present invention relates to a water-insoluble copolymer comprising alkyl(meth)acrylate and methoxypolyethylene glycol(meth)acrylate. The present invention also relates to a method for producing a water-insoluble (meth)acrylate copolymer by copolymerizing alkyl(meth)acrylate and methoxypolyethylene glycol(meth)acrylate in a common polymerization solvent. Further, the present invention relates to a method for purifying (meth)acrylate copolymer by purifying (meth)acrylate copolymer by reprecipitation using a mixed solution of an alcohol having 1 to 10 carbon atoms and water.