Abstract: Back flow blocker in sloping pipes, comprising a well vat (10) provided with an inlet (14) and an outlet (15) to be connected to the pipe. A tube (16) provided within the well vat and being connected at a first end to the inlet and opening at a second end in the well vat is arranged as a floating tube and is pivotally mounted in said first end to be pivoted in vertical direction from an essentially horizontal normal position when water is accumulated in the well vat maintaining said second end above the level of the water in the well vat.