According to development of information technology, a variety of social risks have emerged and complex social problems have happened. To cope with this situation, risk communication is useful. In risk communication, negotiation between participants and decision maker is performed and solution for risks is made. Now we have been developing Multiplex Risk Communicator (MRC) to solve social risk problems and social negotiation problems. In this paper, we propose negotiation subsystem that decides optimal solution among participant and decision maker using non-discrimination curve and utility theory. The effectiveness of proposed method is made clear using an example.