To remove water contained in a weak layer while diluting salt concn. when dredged soil in sea sand is utilized as a building material to enable excavation and transport.

Fresh water is stagnated above dredged deposited soil and, while fresh water is forcibly sucked to the water suction point provided to the lower part of the deposited soil in a gravity direction through the introducing sand column 1 provided to the deposited soil, salt contained in the deposited soil is diluted. After the reduction of salt concn. is confirmed, the stagnation and supply of fresh water is stopped and the forcible suction from the water suction point 2 is continued to dry the deposited soil and a building material not generating turbid water and reduced in environmental pollution is formed.