PURPOSE: To facilitate execution by making adherence of impervious boards provided with impervious sheets on one side of polystyrene resin with water expansive sealant to spread a large number of impervious boards all over the ground of a developed land.

CONSTITUTION: Impervious sheets 4 are adhered to the whole area of one side of a hard and a block-shaped or 21 plate-like polystyrene resin 3 with an adhesive to form impervious boards 2. After that, a nonwoven fabric 9 is spread all over the ground 1 of a developed land, a projection 7 and a recession 8 of adjacent boards 2 are connected to each other in a state to turn the impervious sheets 4 of the impervious boards 2 upward on the fabric 9, and adherence of them is made with water expansive sealants 12 to spread a large number of boards 2. Anchors 10 are driven into the ground 1 through the sealants 12 by passing through the fabric 9 at proper positions of step sections 4, and imperviousness is made by a joint tape 13, an auxiliary sheet 14 and calkings 15. According to the constitution, work can be simply carried out.