Abstract: To clean sediments and the like trapped not only in a top layer of a sand filtration layer, but also in intermediate layers. A seawater infiltration method which uses a water infiltration intake unit which is formed in advance and provided with a water intake pipe embedded in a gravel layer which forms a deep layer of the sand filtration layer, and a backwashing pipe embedded in a sand layer which forms an intermediate layer and a surface layer of the sand filtration layer, and a water suction pipe which is disposed above the sand layer. A desired number of water infiltration intake units are combined to form a sand filtration layer at an installation site on an ocean floor, and they intake seawater from the sea which has undergone natural infiltration in the sand filtration layer and this is introduced into the water intake pipe. The seawater infiltration rate is set at less than 400 m/day.