To ensure the water shielding property by inserting water barrier sheets to an underground water barrier wall groove constructed on a ground, and fitting the adjacent end parts thereof to opening parts on both ends of a connecting tool.

In construction, a cylindrical groove 7a is formed in the part corresponding to a connecting part 10 of a groove 7. A weight 5 is mounted on the lower end of a substantially H-shaped sectional lengthy connecting tool having opening parts 1a, 1b on both ends, and the connecting tool is inserted into the groove 7a so that the openings 1a, 1b are vertical to the side wall of the groove 7. Water barrier sheets 2, 3 are inserted so that respective end parts 2a, 3a are fitted to the openings 1a, 1b, and a water shielding agent is filled in the clearance between the end parts 2a, 3a and the openings 1a, 1b as occasion demands. The inner wall of the groove 7a is covered with a sheet, and mortar 6 is filled in the inner part. According to this, since the end part of the water barrier sheet is fitted to the opening part of the connecting tool, the water barrier sheet can be surely fixed in a prescribed position, the reduction in water shielding reliability of the connecting part between the water barrier sheets can be prevented, and the connecting part can be easily constructed.