To prevent cut-off of underground water flow by a simple method.

In the case of constructing an underground continuous wall, water or muddy water 9 is filled in a bag body 7, and concrete is placed in a hole 2 in the state of cover bodies 6 closing both ends of a water passing pipe 4. After an underground construction is completed, water or muddy water 9 in the bag body 7 is drained, the bag body 7 and a supply-drain hose 8 are removed, and the cover bodies 6 are brought down. A jet nozzle is lowered into the water passing pipe 4 via a connecting pipe 5 to jet high pressure wash water to both sides of the water passing pipe 4. Finally the jet nozzle is pulled up, and water permeable material 14 is filled in the water passing pipe 4 from the connecting pipe 5.