PURPOSE: To efficiently purify seawater within specific sea areas by installing seawater purifiers for filtering and purifying the seawater in a number corresponding to the area of the sea area to be purified and simultaneously enclosing the sea areas to be purified with enclosing means.

CONSTITUTION: Seawater purifiers 1 constructed so as to initially suck seawater with a seawater sucking means installed in each floatable body, filter the seawater through a seawater filter provided on the downstream side of the seawater sucking means and discharge the filtered seawater with a discharging means are installed in sea areas 35 in a number corresponding to the area of the sea areas 35 with anchors, etc. The above-mentioned sea areas 35 are then enclosed with enclosing means such as nets 20 to purify the seawater in the sea areas to be purified with the sea water purifiers 1.