A method for processing a cooling water system having a calcium hardness of 250 mg/L or less in terms of CaCO3, wherein a processing agent that contains a (meth)acrylate copolymer is added. The (meth)acrylate copolymer has a structural unit (a) that is derived from a (meth)acrylate monomer (A) represented by general formula (1) and a structural unit (b) that is derived from a (meth)allyl ether monomer (B) represented by general formula (2). The content of the structural unit (b) is 1-15% by mole in 100% by mole of all the structural units derived from monomers. The (meth)acrylate copolymer has a weight average molecular weight of 7,000-28,000. At least one end of the main chain of the (meth)acrylate copolymer is a sulfonic acid group or a salt thereof.