Disclosed is a therapeutic efficacy potentiator which can be used in the treatment of an infectious disease induced by a bacterium that produces a metal ion-containing peptidase, wherein the therapeutic efficacy potentiator comprises an edetic acid salt. Specifically disclosed is a therapeutic efficacy potentiator which can be used in the treatment of an infectious disease induced by a bacterium that produces a metal ion-containing peptidase, wherein the therapeutic efficacy potentiator comprises an edetic acid compound selected from the group consisting of edetic acid, an edetic acid salt and hydrates thereof.