In this paper, we propose a framework for conversational robots that facilitates fourparticipant groups. In three-participant conversations, the minimum unit for multiparty conversations, social imbalance, in which a participant is left behind in the current conversation, sometimes occurs. In such scenarios, a conversational robot has the potential to facilitate situations as the fourth participant. Consequently, we present model procedures for obtaining conversational initiatives in

more »

... steps to engage such four-participant conversations. During the procedures, a facilitator must be aware of both the presence of dominant participants leading the current conversation and the status of any participant that is left behind. We model and optimize these situations and procedures as a partially observable Markov decision process. The results of experiments conducted to evaluate the proposed procedures show evidence of their acceptability and feeling of groupness.