PROBLEM TO BE SOLVED: To horizontally arrange seepage control sheets for being vertically inserted into water, and enable horizontal ends of them to be connected together by a simply-configured underwater ultrasonic welding machine.SOLUTION: A ditch 2 dug in ground 1 is filled with a stable liquid. Seepage control sheets 3 are vertically inserted into the stable liquid and horizontally arranged. Automatic ultrasonic welding 4 is applied to a connection part in which horizontal ends of the seepage control sheets 3 overlap each other, by an underwater ultrasonic welding machine 7 automatically ascending/descending along guide rails 6 installed in a vertical direction, in the stable liquid of a part in which the horizontal ends of them overlap each other.
