PURPOSE: To simply replace a rail and to enhance the operating efficiency of a basin by forming flights so as to float the same along with a chain by buoyancy and providing a press rail preventing the flotation of the flights when the flights are present on the bottom part of the basin.

CONSTITUTION: Flights 22 are mounted to the chain 21 arranged in a sedimentation basin 1 at a predetermined interval and provided in the basin 1 so as to scrape up the sludge settled on the bottom of the basin toward a pit part 3. At least, the flights 2 are formed so as to float along with the chain 21 by buoyancy. Further, a press rail 5 preventing the flotation of the flights 22 when the flights 22 are present on the bottom part of the basin is provided. As a result, the rail can be simply replaced and the operating efficiency of the basin is enhanced.