To provide a method which enables the separation and removal of excess paint mixed in circulating water in a wet paint booth by coagulation, and enables the circulating water to be recycled well.

In this method, the circulating water W in the wet paint booth 3 is electrolyzed in a circulating water pit 1 by electrodes 4 where Al is an anode, and then treated with electrolyzed Al eluted from the Al electrode and a cation polymer added from a line 54, and the like. Flocs F are generated by coagulating the paint in the circulating water W, and the flocs F are separated by a separator 5. Purified water is returned into the circulating water pit 1 from a line 53.