PURPOSE: To prevent the leak of water as well as increase the strength of an underground structure by a method in which preceding steel plates for forming both ends of elements to be connected are joined with a clamping means consisting of bolt and nut as a connecting means and then concrete is placed.

CONSTITUTION: A sheathing plate 14 is driven on both ends of the element 1 and coupler II of an unit underground wall A in such a way as to surround the coupler II, the ground is excavated, a succeeding steel plate 14 is hung down, and both sides of the steel plate 14 are lapped over the sides of a succeeding steel plate 8 to form the ends of both elements. A hole is then drilled in the lapped part between the preceding and succeeding steel plates 8 and 10 by means of an underwater operator (not illustrated), and cross main iron bars 6 are connected to them by fixing work using a clamping parts 11. Then, concretes 12 and 13 are placed and the horizontal iron bars 6 as the main iron bars are fixed.