Recently, an increase in the load from the land and a food scatter of aquaculture causes eutrophication in IMARI Bay. Therefore, red tides have been generated. It is necessary to decrease the red tide. So, as a basic research, we clarify the characteristics of flow structure and water quality by means of field observations and a numerical model. As a consequence, it is clear that flow structure around bay mouth is very complex. Bay water flows out in the upper layer during ebb tide, and outer sea water flows into the lower layer during flood tide. It is realized that the tidal exchange is strong comparatively in the western part of the bay, while it is weak in the central and south-eastern part.
