A method for supplying binder into a coating tank for electrophoretic power coating comprises: a step for transferring into a solid/liquid separation tank a proper amount of plating bath liquid contained in the coating tank of which the binder component is reduced as a result of the work for coating a given number of articles; a step in which the plating bath liquid is left as it is so as to form supernatant liquid and residual liquid; a step for agitating the supernatant liquid in a supernatant tank; a step for returning the residual liquid to the coating tank; a step for supplying mother binder liquid corresponding in amount to the reduced binder component to a mixing tank; a step for transferring a given amount of the liquid in the supernatant liquid tank into the mixing tank; a step in which a proper amount of plating bath liquid in the coating tank is supplied into the mixing tank while being agitated to form replenisher or supplementary bath liquid; and a step for supplying the replenisher bath liquid into the coating tank. The method is realized by an apparatus of this invention for supplying binder into a coating tank for electrophoretic powder coating.