To install a drain simply by continuously mounting water-permeable materials beforehand filled with soluble fillers along the external wall surface of an underground ditch, placing concrete into the underground ditch and removing the fillers from the water-permeable materials.

The voids of water-permeable materials 10 consisting of a material having a large number of voids are filled with soluble fillers composed of a biodegradable high-molecular material such as CMC (sodium carboxymethylcellulose) or a natural paste material or the like. The water- permeable materials 10 are installed continuously in the horizontal direction along the external wall surface of an underground ditch 1 while reinforced cylinders 2 are mounted. Concrete is placed into the underground ditch 1, and solidified, thus forming a cast-in-situ diaphragm wall. When the soluble fillers are dissolved and removed from the insides of the water-permeable materials 10, a large number of voids in the water-permeable materials 10 are recovered, and a beltlike drain continuing in the horizontal direction is formed on the outside of the cast-in-situ diaphragm wall. Accordingly, since the flow of groundwater dammed up by the diaphragm wall can be bypassed to the side, the rise of a groundwater level can be prevented.