The invention provides an osmosis filtering method for sea water and an osmosis water intake unit to cleanse a suspended matter or the like captured on the surface of a sand filtering layer as well as on an intermediate layer. In the osmosis filtering method for sea water, an osmosis water intake unit (11) is formed beforehand in which a water intake pipe (12) is embedded in a gravel layer (13) that is for forming a lower layer of a sand filtering layer, a reverse cleaning pipe (14) is embedded in a sand layer (15) that is for forming an intermediate layer and a surface layer of the sand filtering layer, and a water suction pipe (16) is installed above the sand layer (15). A sand filtering layer is formed by combining a required number of osmosis water intake units (11) at an installation location on the sea bed. Sea water is taken in when sea water that naturally permeates through the sand filtering layer is introduced into the water intake pipe (12). The sea water permeation speed is no faster than 400m/day. A suspended matter or the like held in the intermediate layer of the sand filtering layer is agitated by injecting water or air from the reverse cleaning pipe (14), the suspended matter is blown onto the surface layer of the sand filtering layer and recovered by suctioning the agitated water from the water suction pipe (16). [Effect] The seawater permeation speed can be maintained at the highest speed possible, i.e., no faster than 400m/day.