PURPOSE: To provide a water barrier sheet especially laid on a landfill site and keeping excellent waterproof function preventing the penetration of water by closing the hole made in the sheet by the penetration of foreign matter into the sheet from above or below in a moment and not damaged by the edge of a rock.

CONSTITUTION: In a water barrier construction method adapted to a waste final disposal plant, a waterproof sheet layer 2 is laid on foundation 5 and a two-pack type reactive curing type urethane film material reacting a prepolymer having an isocyanate group at its terminal and a polyol-containing compd. is applied to the waterproof sheet layer 2 by spray coating to form a seal layer 4 and a protective layer 3 is further laminated on the seal layer 4.