Abstract: The present invention relates to a method for replenishing an electrodeposition coating bath by subjecting to an ultrafiltration, an electro-deposition bath liquid which has been decomposed with time under a pressure of 0.2-3.0 kg/cm.sup.2 changing the direction of the bath liquid during the ultrafiltration step so as to cause the liquid to flow in opposite directions in the ultrafiltering apparatus; recycling the concentrate obtained from the ultrafiltration to the electrodeposition coating bath for the purpose of adjusting the liquid characteristics of the bath and using the thus-obtained filtrate, obtained from the ultrafiltration, the diluent for preparing a replenishing supply liquid.