PURPOSE: To efficiently execute separation and collection of sludge by mixing sludge separation chemicals into dirty water in a water tank, separating water and sludge and sedimenting the sludge, and taking out the sludge after water is eliminated by a pump, etc., while stirring it with a stirring implement.

CONSTITUTION: After dirty water is contained into a water tank 1, sludge separation chemicals 6 are thrown into this dirty water, water 2 and sludge 3 are separated by mixing them enough, and the sludge 3 is sedimented downward. In such a state, first of all, the water 2 separated to the upper position is eliminated to the outside by utilizing a pump, etc., and thereafter, an opening/closing valve is opened, and also, a stirring implement 4 is rotated, and the sludge 3 separated and stored in the lower position in the water tank 1 is taken out of an outlet opening in the lower part. In such a way, according to the method for eliminating the sludge after eliminating the separated water, the sludge minimized in water content can be taken out efficiently.