PURPOSE: To perfectly prevent a back flow to a house waste-water facility by sensing the back flow to actuate a drain pump so that a drainage is sent to a waste-water vessel.

CONSTITUTION: In a time zone where pitchen waste-water is generated concentrically, the waste-water amount flowing a vertical pipe 1 increases so that the water pressure in the vertical pipe 1 on a lower floor exceeds that at a branch pipe 2. When the waste-water flows back in the branch pipe 1 as far as a house waste-water facility 3 side, a back-flow detecting device 6 detects it. A back- flow detection output issued from the backflow detecting device 6 actuates a drain pump 9 provided to a branch pipe 12 between the bottom of the vertical pipe 1 and an waste-water vessel 4, causing a drainage to be sent to the waste- water vessel 4.