An automatic excretion disposal device which is configured to automatically perform an excretion disposal of a person to be cared for, the device comprising: a cup which is configured to be attached to a body of the person to be cared for to receive excretion; a detection unit which is configured to detect excretion in the cup; a tank; a feed water pipe which is configured to decompress cleaning water from a water supply to a predetermined pressure and supplies the cleaning water to the cup and the tank, on the basis of a detection result of the detection unit; a suction motor which is configured to suck sewage generated when the body and an inside of the cup are cleaned with the cleaning water supplied from the feed water pipe, and to discharge the sewage to outside of the cup and; the tank is configured to temporarily store the sewage sucked by the suction motor; a drainage pump connected to the tank which is configured to cause drainage which is a mixture of the sewage stored in the tank and the cleaning water directly supplied from the feed water pipe, to forcibly flow out to a drainage piping connected to a sewer or a septic tank, while pulverizing solid matters contained in the drainage, the drainage piping comprises a drainage pipe connected to the drainage pump; an air supply pipe which is configured to supply air sucked by the suction motor to the cup; characterized by a branch pipe which is configured to branch from the middle of the air supply pipe and is connected to the drainage pipe; and a collecting unit which is configured to connect the branch pipe and the drainage pipe, exhausts a positive pressure air generated in the air supply pipe to the drainage pipe, and is formed to generate a back pressure which assists outflow of the drainage flowing out to the downstream side of the drainage pipe. a cup which is configured to be attached to a body of the person to be cared for to receive excretion; a detection unit which is configured to detect excretion in the cup; a tank; a feed water pipe which is configured to decompress cleaning water from a water supply to a predetermined pressure and supplies the cleaning water to the cup and the tank, on the basis of a detection result of the detection unit; a suction motor which is configured to suck sewage generated when the body and an inside of the cup are cleaned with the cleaning water supplied from the feed water pipe, and to discharge the sewage to outside of the cup and; the tank is configured to temporarily store the sewage sucked by the suction motor; a drainage pump connected to the tank which is configured to cause drainage which is a mixture of the sewage stored in the tank and the cleaning water directly supplied from the feed water pipe, to forcibly flow out to a drainage piping connected to a sewer or a septic tank, while pulverizing solid matters contained in the drainage, the drainage piping comprises a drainage pipe connected to the drainage pump; an air supply pipe which is configured to supply air sucked by the suction motor to the cup; characterized by a branch pipe which is configured to branch from the middle of the air supply pipe and is connected to the drainage pipe; and a collecting unit which is configured to connect the branch pipe and the drainage pipe, exhausts a positive pressure air generated in the air supply pipe to the drainage pipe, and is formed to generate a back pressure which assists outflow of the drainage flowing out to the downstream side of the drainage pipe.