To more well discharge only supernatant water.

Supernatant water W separated by sedimenting activated sludge is sucked and discharged by a pump through a drain pipe 12 and a flexible hose 20. The drain pipe 12 is attached to the lower part of a float 16 in such a state that the inflow port 14 thereof is turned toward the surface of the water. A closure element 24 displaced up and down by the driving of an air cylinder 28 to open close the inflow port 14 is provided to the float 16.