PURPOSE: To prevent foreign materials from flowing in a U-shaped passage by closing the upper end of the U-shaped passage forming member by means of a cover member forming a hanging wall, setting the lower end of the hanging wall below the upper opening end of the U-shaped passage, and communicating the upper opening end with the outside via a space defined by the hanging wall and the U-shaped passage.

CONSTITUTION: A level difference between a pumping start water level RWL and a pumping stop water level SWL is properly set by adjusting the levels of the upper end opening portion 12 of a U-shaped passage 21 and the returning point 13 thereof, so that the occurrence of a hunting phenomenon can be prevented. Although a water level rises from a level lower than that of the upper opening end 12 of the passage 21 to the same level as that of the upper opening end 12 and water flows in the passage 21, since the lower end of the hanging wall 35A of a cover member 35 is below the upper opening end 12, foreign materials floating on a water surface can not flow in the passage 21 together with water. Consequently, a good flowing condition in the passage 21 is obtained to avoid the lowering of the performance of pumping start and pumping stop.