To improve an adhesive property and workability by applying an instantaneous adhesive to a joining margin when joining a waterproof sheet, and specifying an application quantity of the adhesive.

A joining margin 1a is joined in layers after applying an instantaneous adhesive 2 to the joining margin 1a of adjacent sheets 1 by setting an application quantity to 50 to 250 g/m2 when laying the waterproof sheets 1 such as plural rubber sheets on the ground. In this case, since the adhesive 2 is immediately stuck together after being applied to the joining margin 1a, dust does not stick to the adhesive 2, so that an adhesive property is not hindered by the dust. Next, reduction in adhesive strength by degrading when moisture acts on the adhesive 2, can be prevented by applying a seal material 3 along a part between the edge of the joining margin 1a of the upper side sheet 1 and the upper surface of the lower side sheet 1 when necessary. Therefore, initial adhesive strength can be enhanced, high adhesive strength can be secured even by a small quantity of adhesive and small rolling, workability can be reduced, and water resistance can be also improved.