PURPOSE: To prevent the loading of a vertical draining material by confining specific flocculant in a continuous bag having small water permeability.

CONSTITUTION: A flocculant powder 1 composed mainly of aluminum sulfate is confined in a continuous bag 2 made of a water-resistant and hardly water-permeable sheet with a large number of needle holes. The bags 2 are attached to both sides of a water guide plate 4 having grooves 3 on its upper and lower sides, and the outside is covered with a long cylinder 5 made of a filter material to form a vertical draining material A. When the vertical draining material A is used, the flocculant powder 1 in contact with void water in the ground is gradually dissolved in the void water to cause fine soil particles to flock by static electricity.