This paper describes a strategy for the conversation system to take part in human-to-human group conversation. One big characteristic of the group conversation system is that it can choose whether to observe or to take turn in the conversation. We implement the computational model combined with speech and gaze recognizers to keep the rules in turn taking, and define an interruption decision strategy based on an analysis of human needs. And finally, we realized a human-friendly group conversation system by combining multi-modal information processing/expression abilities of humanoid robot ROBITA.