PURPOSE: To make it possible to prevent the lack of uniformity of an underground wall surface resultant from the collapse of an inner wall surface and finish the wall surface with high accuracy without executing troublesome work.

CONSTITUTION: A ditch 3 is excavated while muddy water 2 is being supplied. A partition board 4 is suspended along an inner wall surface of the ditch 3. A cut-off sheet 5, which may produce a relatively sliding motion between the partition board 4 and the ditch 3, is laid out between the partition board 4 and the ditch 3. The muddy water 2 is replaced at an inside spot partitioned by the partition board 4 while concrete C is being cast, thereby building up an underground wall. After the concrete C is cured, the partition board 4 is pulled off.