In closed sea areas in the world the eutrophication is being progressed day by day. Nowadays in closed sea areas in Japan it is hard to control the seawater quality in deep areas because of the poor oxygen seawater or the anoxic seawater. Aerobic microorganisms can contribute to decompose organic compounds in the seawater as long as they live to consume oxygen. As a result the oxygen decreases in deep areas. It is necessary to maintain that the seawater is clean and rich in nature for the sustainable development. One of methods is this sustainable seawater purification system built in quays and piles using purifying functions of microorganisms and the tidal energy (Dan et al. 2017). It is shown that this system can decrease Chemical Oxygen Demand (CODMn) in the seawater experimentally and can be utilized in order to purify the seawater from the depth to the shallows using this system built in piles partially around a pile and using this system built in quays widely along a quay. This system has following advantages. 1.Using the tidal energy –> “ecosystem” 2.Using the purifying functions of microorganisms, decomposing organic materials –> “ecological and natural without chemicals”, 3.Capable of purifying the seawater in the shallow area, especially also in the deep area –> “useful” in closed sea areas 4.This system built in piles or quays is simple. Not to construct new quays but to construct the purifying room additionally. It costs less. –> “economical” The initial CODMn is almost 8 by putting sugar adequately in seawater and mixing (Fig. 9 and 10). The changing process of CODMn is checked by measuring CODMn according to JIS K 0102 (Japanese industrial Standards). In the case of seawater purification system CODMn is becoming smaller and it is found that this system can reduce CODMn in seawater. In future in order to construct this system built in piles or in quays in the field following procedure is considered, 　①When the field site to construct this system is selected and the tidal range z is given. 　②Because the velocity v in the vessel through gravels is sufficiently small for microorganisms to decompose organic compounds then v