As the water pollution in the bay and in the coastal lake which is used as a fish farm advanced, a countermeasure plan is proceeding to conduct freah sea water into the lake through a pipe. In this paper, the authers have given the fundamental equations in which the variations in concentration of pollutant and the displacement of the water level of the lake was expressed, and have numerically investigated after rearranging these equations with dimentionless parameters. subsequently, hydrauric model experiment have been made to confirm this equations. Although the variation in concentration of the lake water is characterized by several dimentionless parameters, among these parameters, D^2/A, l/D, l/h, (D: pipe radius, A: surface area of the lake at a average depth, l: the length of pipe, h: average depth of the lake,) are the most important parameters that influence on the concentration variation of the lake. As will be shown later, there exists a certain value for D^2/A where a drop in concentration has a maximum corresponding to given values for l/D, l/h, the most suitable design for the pipe can be determined according to these values.