A liquid treating device without any restrictions for a sludge discharge device set above a liquid tank is provided. A cyclone separator adapted to swirl liquid to subject sludge to centrifugal separation from the liquid and precipitate the sludge is connected to the circulating pump adapted to pump up liquid coolant containing the sludge discharged to a liquid tank from a machine tool. An opening/closing valve for sludge discharge is arranged at a discharge port of a sludge pot adapted to condense the sludge precipitated by the cyclone separator. A sludge discharge device to discharge only the sludge to the exterior from a sludge storage tank part adapted to store the sludge and liquid discharged from the sludge pot via the opening/closing valve temporarily is set. A supernatant liquid stored in the sludge storage tank part of the sludge discharge device in the suction side of the pump is recovered.