Nowadays, the conflict between people insisting on environment and people insisting on development comes to be seen frequently. Management of such conflict and inducing consensus between them must be considered on a future development project. Without such understanding, appropriate development would not be achieved at an appropriate time. Besides, opinions of residents should be considered more to development project in the future, and it might become impossible to promote project without considering their opinions. They are so various that more conflicts might be occurred if they come to insist on their rights. Therefore, the conflict incidental to development project should be regarded as interactive phenomenon of conflict situation and residents' opinion between development and environment. From above-mentioned recognition, it is needed for the future development project to manage the conflict as taking various opinions of residents into consideration. Therefore, first of all in this paper, people concerned to the conflict are defined as groups categorized by some features and assumed to have distribution of opinions. A categorized group is called a player in this study. The distribution of opinions and the changing process of it are modeled by ‘synergetics’. The concept of ‘synergetics’ is that it is science treating static or dynamic mass phenomenon that is shown in a close system or open system consisting of many factors that interact each other. That is, ‘synergetics’ formularize the dynamism of the society under the assumption that the society stands up on interactive phenomenon. Secondly, the method is shown for deciding preference vectors using opinion distribution, mentioned above. The preference vector is the order of preference of each player for a set of possible consequences, each of which is a combination of each player's option, and this is the input of ‘replicator dynamics’. ‘Replicator dynamics’ is described with differential equation. The concept of it is based on ‘evolutional game theory.’ An option that is indicated in the equilibrium solution is supposed to be a behavior that is actually taken by a player. Through the model analysis, it can be seen how the conflict would reach stable states. Furthermore, our model is applied to Nagara River Weir Problem as a numerical example. Nagara River Weir Problem had been under conflict situation for 30 years in Japan. In this study, the conflict between central government and basin residents, which occurred from about 1990 to 1994, is applied to our model as 2 players game.