PURPOSE: To surely discharge supernatant in the basin following up water level in settling basins, etc.

CONSTITUTION: A fixed waste pipe installed in the lower end of a flexible waste pipe equipped with a catchment trough 4 which sinks and floats following up water level by a float 3 is bent in the U-shape to lead water from the upper part of the basin to outside the basin. At the same time, a water supply valve 10 and a suction and exhaust valve 12 for generating and cutting off siphonage in the fixed waste pipe respectively are provided. Thereby, even when water level is changed and the upper end of the drain hose with the catchment trough is moved up and down by starting drainage, a control type feed cable and hose is not damaged because of being not moved and even if a flexible drain hose 2 is broken down, the control system is operable which can stop drainage by cutting off the siphonage to prevent the discharge of untreated effluent and the discharge of precipitated sludge.