Incident analysis is an important activity to maintain the safety of nuclear power plants. Much discussion is required to utilize the collected incidents effectively in the incident analysis activity. On-line Computer Mediated Communication(CMC) activity is an appropriate circumstance for geographically dispersed workers in nuclear power plants to discuss about the incident analysis. Some studies, however, indicate that the discussion activity in CMC tends to stagnate after a short period of time. For this study, the authors developed a discussion system for knowledge sharing and collaborative analysis of incidents, and proposed a method to promote discussion among users through introduction of "Active Participant". The Active Participant always behaves actively, and such behaviors are expected to promote the other members' incident analysis in the discussion group. To evaluate the effectiveness of the introduction of the Active Participant and obtain concrete guidance of the Active Participant, an experiment was conducted with nuclear power plant workers who were asked to evaluate the discussion system. The results of the experiment show that Active Participant can promote discussion among group members if enough number of incidents are submitted to the discussion system.