To reduce the pollution load caused by Combined Sewer Overflow (CSO), which significantly affects the properties of water for public use in the event of rain, effective measures are adopted after the outflow behavior and pollution load are carefully studied. In this research, an area of about 127 ha around the confluence zone located in Nagaoka City, Niigata was targeted, the outflow rate, the water quality of CSO and the amount of rainfall were continuously monitored for 1200 days. The observation was started in June 2004 at two points where chamber has been equipped. As a next step, a simulation model with high reproducibility was designed on the basis of the data obtained by continuous observation using MOUSE. Since the water quality was predicted using the model in the case when it could not be measured, the pollution load of CSO was accurately estimated. It was found that the pollution loads of SS and of BOD for eight months (from April to November) were 50×10³ kg and 28×10³ kg, respectively. In addition, we found that the pollution load basic units of SS and BOD were 59×10³ kg km^-2 year^-1 and 33×10³ kg km^-2 year^-1, respectively.