PURPOSE: To continuously take out a highly concentrated sludge under a stable condition in the concentration of a sludge by a compression-floatation-concentration method, by controlling a scraped amount of a floating sludge so as to maintain the downside surface of a floating sludge zone at a predetermined level.

CONSTITUTION: If a lower boundary gauge 3 detects the downside surface 2a of a floating sludge zone 2, a motor 6 is rotated by a controller 7, and a scraper 5 is operated to scrape out a part of a floating sludge. With the advancement of the scraping, the downside surface 2a of the floating sludge zone 2 is raised. The raise of the downside surface 2a is detected by an upper boundary gauge 4, so that the rotation of the motor 6 is stopped by the controller 7 and that the downside surface 2a of the floating sludge zone 2 is lowered. The downside surface 2a of the floating sludge zone 2 is positioned between the lower boundary gauge 3 and the upper boundary gauge 4 by the control of the amount of the floating sludge to be scraped out. That is, the downside surface 2a is maintained at a predetermined level.