PURPOSE: To thicken quickly and highly sludge water without using mechanical power by utilizing the capillarity of a fiber bundle in separating sludge and water.

CONSTITUTION: The one side of a fiber bundle 4 is installed on the thickner 1 side and the other one side is installed in a release part 5 in thickening sludge water such as waste water from sewerage, water works and factories. The water level in the part 5 is designed lower than the water level in the service water part 3 of the tank 1 to utilize a siphon phenomenon. The sepn. of the service water 3 and sludge 2 in the tank 1 is accomplished in the same way as in the prior art. and the effect of increasing the cleaning up treatment by the capillarity of the bundle 4 is added. The sludge water is highly thickened without requiring energy by the above-mentioned method.