PURPOSE: To provide a waterproof layer with high water preventing characteristic by accurately controlling the degree of swelling through the constitution where one of upward and downward layers closed in their circumference is made waterproof, and the sheets for the two layers are connected by a space adjustment member, and bentonite is filled in the bag formed in this way.

CONSTITUTION: Two upper and lower layers of bentonite waterproof mat 6 are closed by sheets 7, 7 of polyester fiber woven fabric at their circumference and the part between the sheets are connected in a lattice shape by a space adjustment member 8. Next, in the bag formed in this way bentonite powder 9 is stored. The waterproof mat 6 is layed on a natural ground face 10 and water is supplied through the water permeation sheet 7 to bring the powder 9 into a hydration swelling condition. The degree of swelling is restricted by the space capacity between the two layers and regulated to form a swelling layer. It is thus possible to work a bentonite hydro-swelling layer with high water-preventive characteristic efficiently without any need of earth and the like on the mat 6.