An endoscope cleaning/disinfecting apparatus includes an endoscope connection section, a liquid supply conduit, a gas feeding conduit, a first gas feeding section, a first exhaust section, and a control section that performs a step of filling the gas feeding conduit with a cleaning liquid, a step of opening the first exhaust section, a step of allowing a gas to flow from the first gas feeding section to the endoscope connection section and feeding a two-phase gas-liquid fluid with which the gas feeding conduit is filled into the conduit of the endoscope and a step of closing the first exhaust section a predetermined time after allowing the gas to flow.