Deterioration of water quality is a serious problem in the outer moats of the YEDO castle due to CSO (Combined Sewer Overflows). People who live in the area are suffered from stench and landscape destruction. The purpose of this study was to grasp making of ecosystem model and the properties of nutrient and chlorophyll-a movement in the outer moats of the yedo castle. As a result, the concentration of chlorophyll-a in the summertime were beyond the numerical target for 42 days. But because there were only 27 days beyond the numerical target by movable weir introduction．And the chlorophyll-a concentration could make them reduction 58 ％ more than the current situation by movable weir introduction.