PURPOSE: To enhance the efficiency of the sedimentation and filtering of the next process while holding function as a floc forming basin, by together providing a sludge scraper and a flocculator to the bottom of the floc forming basin.

CONSTITUTION: A sludge scraper is arranged to the bottom of the floc forming basin of a final row in combination with a flocculator to attain the growth of floc and, at the same time, the sludge accumulated on the bottom of the basin is scraped up continuously to be removed out of the basin to hold the function of the floc forming basin. That is, the sludge scraper 1 is arranged to the lower region of the flocculator 11 in the basin 21c of the third row of the flock forming basin 21 and power is transmitted to the drive shaft 4 piercing through a wall 10 by the drive apparatus 2 arranged in the drive chamber 9 provided to the bottom part of the basin 21 to operate the sludge scraper 1 in the basin 21. In order to prevent the leakage of water when the drive shaft 4 is pierced through the aforementioned wall 10, an end surface slide type shaft water seal apparatus 12 is attached to the wall 10.