PURPOSE: To facilitate the construction of underwater walls by a method in which flexible bags are set between plural piles driven into the bottom under water until the bottom is covered, and sand is packed into the bags by replacing water in the bags.

CONSTITUTION: Plural piles 1 each of which has guide plates 2 on its left and right sides are driven into the bottom 4 under water at a given interval by facing their guide plates 2. Flexible bags 5 filled with water are set through the guide plates 2 between the piles 1 until the bottom 4 is covered. Two pipes 6 and 7 are inserted into the bag 5, sand 8 is supplied from one pipe 6 into the bag 5, and water in the bag 5 is discharged from the other pipe 7. The sand 8 can thus be easily packed into the bags 5 and underwater walls can be constructed easily and economically.