PURPOSE: To prevent the quality of water from being deteriorated caused by the falling of a part of scum into an overflow water channel together with a supernatant water, by arranging both members in such a positional relation that the longitudinal direction of a scum trough makes an angle to the side of the rotating direction of a scum scraping board.

CONSTITUTION: A scum, floating on the surface of a raw liquid fed to a sedimentation tank, is scraped and gathered to the front of a scum trough 11 by a rotating scum scraping board 10 and is carried into the trough 11 when the board 10 crosses the trough 11. At this time, since the trough 11 is arranged to intersect the board 10 at an angle α, the surface layer of the liquid is moved in the direction of a rotary axis i.e. to the inside while being held between the trough 11 and the board 10, and the scum is also moved to the inside with the movement of the surface-layer liquid. Accordingly, the scum is not pushed out to the side of an overflow channel 4 across a fence 13, and the scum housed in the trough 11 is flowed and discharged through the nearest scum discharging pipe by the flow of a small amount of the liquid which overflows into the trough 11.