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. The deposition bath according to the present invention contains a water thinnable cationic binder resin which is a nitrogen atom-containing basic resin neutralized at least partially with an acid compound and at least one non-ionic resin in the form of a powder which is solid at room temperature, but which is capable of melting, when heated. According to the present invention, the harmful substances are removed from the bath, and the replenishing may be carried out continuously without any serious clogging of the apparatus to produce excellent coating films.