PURPOSE: To prevent the inflow of waste mud into an overflow channel and to prevent the clogging of a filter by discharging waste water from small holes provided on the base of a flow passage in the form of a shower toward nearly right thereunder into a settling chamber.

CONSTITUTION: While the waste water flowing into the flow passage 14 of a mud feed reservoir 6 from an inflow gutter is decelerated in flow velocity by many water stopping plates 19, the water flows from the start point of the flow passage 14 toward the end point. The waste water is discharged nearly right under the many small holes 20 provided on the base 15 of the flow passage 14 between the water stopping plates 19 in the form of shower into the settling chamber by these small holes. The finally remaining waste water flows out of a discharge hole 21 into the settling chamber. Then, the soaring up of the waste mud settled on the base of the settling chamber and the drifting of the settled waste mud are obviated and, therefore, the clogging of the filter 13 is prevented and the exchange time of the filter 13 is prolonged.