PURPOSE: To prevent concrete from alowing to the back side of interlocking pipes placed at the ends of a groove formed in the ground by packing a backing material container bag in each of the gaps beetwen the pipes and the back face soil wall.

CONSTITUTION: In the construction of an underground earth-retaining wall, underground structure wall, or the like a vartical groove 5A of a predetermined depth is formed in the ground, steel interlocking pipes 1 circular in cross section are placed at the ends of the groove 5A, and bags 2 made of a flexible material such as vinyl are placed along the back faces of the pipes 1 to hold the pipes 1 upright. Then a backing material 3 such as gravel is filled in the bags 2, and concrete 7A is cast in the space surrounded by the pipes 1 and the side faces 6 of the groove 5A. After curing of the concrete, the pipes 1 are drawn out and the adjacent spaces are filled with concrete 7B. These steps are repeated to form an underground wall.