To provide a method for preventing microorganism propagation in a pole liquid which is capable of preventing clogging of a diaphragm and clogging in a pole liquid circulating route by preventing the propagation of the microorganisms in the pole liquid, is capable of maintaining electrodeposition coating efficiency as a result thereof and is capable of assuring the coating film quality of a material to be coated by stabilizing the coating material composition in an electrodeposition tank.

This method for preventing the microorganism propagation comprises adding hydrogen peroxide to the pole liquid in such a manner that the concentration of the hydrogen peroxide in, for example, the pole liquid attains 20 to 1,000 ppm in using an electrodeposition coating equipment having a diaphragm device which is disposed in the electrodeposition tank and contains an electrode immersed in the pole liquid and a pole liquid tank which is disposed outside the electrodeposition tank, stores the pole liquid recovered from the diaphragm device and delivers the pole liquid again to the diaphragm device after regulating its electrical conductivity.