A method for treating circulating water circulating through a wet paint booth, the method comprising coagulating the water-based paint and/or a solvent-based paint included in the circulating water by adding an agent to the circulating water, wherein said agent comprises a phenolic resin having a weight-average molecular weight of 5,000 to 100,000, the phenolic resin serving as an active component, wherein the molecular weight is measured as set out in the description, wherein the phenolic resin is dissolved in an aqueous alkaline solution, wherein the phenolic resin is at least one selected from the group consisting of a novolac-type phenolic resin and a secondary-reaction phenolic resin, wherein said secondary reaction phenolic resin is produced by adding an aldehyde to an aqueous alkaline solution of a novolac-type phenolic resin produced by a reaction of a phenol with an aldehyde in the presence of an acid catalyst and subsequently conducting a resole-type secondary reaction in the presence of an alkali catalyst, wherein said agent is added to the circulating water such that an amount of the phenolic resin is 1 to 200 mg/L, wherein an aluminum-based coagulant and/or a cationic polymer is further added to the circulating water, wherein the phenolic resin when added to water circulating through a wet paint booth dissolves in the circulating water or disperses in the circulating water to form colloidal particles, and wherein the dissolved or dispersed phenolic resin particles are coagulated and insolubilized due to the contribution of the cationic polymer and the aluminum-based coagulant that are included in the circulating water together with the phenolic resin. the method comprising coagulating the water-based paint and/or a solvent-based paint included in the circulating water by adding an agent to the circulating water, wherein said agent comprises a phenolic resin having a weight-average molecular weight of 5,000 to 100,000, the phenolic resin serving as an active component, wherein the molecular weight is measured as set out in the description, wherein the phenolic resin is dissolved in an aqueous alkaline solution, wherein the phenolic resin is at least one selected from the group consisting of a novolac-type phenolic resin and a secondary-reaction phenolic resin, wherein said secondary reaction phenolic resin is produced by adding an aldehyde to an aqueous alkaline solution of a novolac-type phenolic resin produced by a reaction of a phenol with an aldehyde in the presence of an acid catalyst and subsequently conducting a resole-type secondary reaction in the presence of an alkali catalyst, wherein said agent is added to the circulating water such that an amount of the phenolic resin is 1 to 200 mg/L, wherein an aluminum-based coagulant and/or a cationic polymer is further added to the circulating water, wherein the phenolic resin when added to water circulating through a wet paint booth dissolves in the circulating water or disperses in the circulating water to form colloidal particles, and wherein the dissolved or dispersed phenolic resin particles are coagulated and insolubilized due to the contribution of the cationic polymer and the aluminum-based coagulant that are included in the circulating water together with the phenolic resin.