In this study, we analyze a dynamic optimal policy and evaluate new technologies to improve the water quality of a lake, considering both the total ecological system in and around the lake and situational changes over a certain period of time. Three sub-models (the socio-economic model, the material flow balance model and the water cycle model) and one objective function were specified in order to analyze optimal policies to improve the water quality of Lake Kasumigaura. The socio-economic model describes the social and economic activities in the catchment area and the relationship between the activities and the emission of pollutants. The material flow balance model describes change in the amount of generation and flow of pollutants in the lake and the rivers. The optimal policies were derived so as to maximize the objective function (GRP) subject to the structural equations that describe both the ecosystem and socio-economic system. The pollutants measured in this study were nitrogen, phosphorus and COD which strongly affect the deterioration of water quality. JEL classification: Q25, Q55, Q58