PURPOSE: To prevent staying of sea water in a closed water area by providing an inflow side nonreturn valve and an outflow side nonreturn valve on the lower end of an inflow water path and the upper end of an outflow water path, respectively of a closed water area partitioned from the sea area by a partition wall.

CONSTITUTION: An inflow water path 2 is disposed near the lower end of one partition wall 9 of a closed water area 1, and a flap valve type inflow side nonreturn valve 4 which permits inflow of sea water of an outer sea area 11 into the closed water area 1 and prohibits flowing in the reverse direction is provided on the closed water area 1 side. A flow adjusting gate 3 is disposed on the outer sea area 11 side of the inflow water path 2. An outflow water path 5 is disposed near the upper end of the other partition wall 10, and a flap valve type outflow side nonreturn valve 6 is disposed to permit outflowing of sea water of the closed water area 1 into the outer sea area 11 and prohibit flowing in the reverse direction. By this arrangement, the quality of water in the closed water area 1 can be maintained at a low power cost.