This paper describes an overview of an online training support system for ADR mediators. To educate good mediators, much training is necessary, which is not easy for supervisors. As supervisors have to take care of many students, they cannot spare much time for a specific student. To train students effectively, some support system is needed.This system provides an environment for online disputation. Using this system, the supervisor and students can participate in the mediation process even if they are outside of the University. Furthermore, this system stores many disputation records in the form of XML documents as a case base, and this case base is used to navigate the mediation process. During the disputation, users can retrieve old similar scenes of disputation, and they can construct proper arguments by referring to similar scenes. Furthermore, by comparing records of disputation or by analyzing them statistically, we can get the information that help to evaluate the mediation skill.