PURPOSE: To maintain the strength of coupling portions as well as to raise the working efficiency by a method in which coupling pillars consisting of a combination of a semi-tubular steel material and a partition plate are combined in such a way as to widen their joints and also to enable foreign matters, e.g., soil, etc., to be easily removed.

CONSTITUTION: A combination of an iron bar cage 4 and a coupling pillar 5 for a preceding wall 2 is erected in a given place in a trench 1, cement mortar 56 is packed between a semi-tubular steel material 51 and a sheet 54, and the coupling pillar is fixed to the trench 1. Concrete is then placed into the trench 1, the ground is excavated for a following wall, and soil and sludge adhered to the coupling pillars 5 of the following wall side are removed by a cleaner. An iron bar cage 4' is erected in the trench 1 to construct an underground wall.