To provide both a cut-off wall-installing well constituted to be adhered to in-situ rocks of a water permeable layer appropriately to secure desired ground water treatment and a ground water treating mechanism for cut-off walls to abolish a pumping well in an excavated part by utilizing the cut-off wall-installing well and to connect the upstream side to the downstream side of a watertight wall by a continuing pipe.

A cut-off wall-installing well 1 is constituted of a container 3 which is provided with a water collecting pipe 2 and in which a front surface opening is constituted of a filter, a front surface cover 4 provided with a freely detachable cutting rib 8 to block the front surface of the container 3, and an extendable spacer 5 with a spring arranged on the rear surface of the container 3. An excavating surface into which cement is penetrated is scraped by the cutting rib 8, and the cut-off wall-installing well 1 is adhered to in-situ rocks of a water permeable layer to enable the collection and injection of ground water to secure a stream of ground water prior to the installation of cut-off walls. In addition, in a ground water treating mechanism, by making a cut-off wall-installing wall arranged at a water permeable section of a cut-off wall on the side of in-situ rocks be a pumping wall and connecting the cut-off wall-inserting wells to each other, an equivalent water level on upstream side and downstream side of the cut-off wall is maintained.