PURPOSE: To make it possible to raise and lower an arm surely by a simple structure through a mechanism wherein an arm on a moving cart on a precipitation basin is supported to a lower side rail during scraping and supported to an upper side rail during returning to separate the scraper of an arm from a bottom of the precipitation basin.

CONSTITUTION: A running cart 1 on a precipitation basin 2 is moved upto a left end of a figure. In this time, wheel 9 positioned to an intermediate point of an arm 6 hanged from a cart has been supported on a lower side rail 8a of an upper and lower two stages rail along a moving direction of the cart. In this condition, if the cart is moved, a scraper 7 provided to the arm is moved while contacted with a basin 2a to scrape the sludge 16 to a sludge pit 10. Moreover, the wheel 9 is raised along a bent portion of an end of the rail 8a by cart movement and transferred onto an upper surface thereof by kicking up a switch rail 12. Thereby, the scraper is raised from a basin bottom and the wheel 9 is returned while moved on a rail 8b by reversely running the cart. When the wheel 9 is reached to a switch rail 14, said wheel 9 is swung downwardly and supported on the rail 8a from an upper portion thereof.