PURPOSE: To purify the water quality on an internal water surface by a method wherein sea water is caused to efficiently flow through an internal water surface near a sea shore by utilizing a tide.

CONSTITUTION: First and second weirs through which an internal water surface is communicated with a sea surface are built separately away from each other. A first weirs 5a is wide and an upper end position is set to a value high than the lowermost water level of a tide. Further, a second weir 5b has width set to a value lower than that of the first weir 1 and moreover, the upper end position thereof is set to a value lower than that of the first weir 1.