This paper is based on a case study investigating on the process and the mechanism of the public participation in regional environmental planning in Sanbanze, Chiba, Japan. The Sanbanze regional environmental planning project was performed within a span of two years from January 2002 to January 2004. It is the biggest round-table conference for the public participation and the first attempt of plan-making by citizens in Japan. This paper discusses the issues of public participation by focusing not only on the committee members, but also on the observers. Interviews and questionnaire surveys were carried out. The results show that although the very intensive participation process was conducted, more people can participate in the process of planning and designing for Sanbanze. However, some problems such as the imbalance of observers attributes and the gradual decrease of the number of observers appeared. Our analysis shows some structural factors also have a deep relation with observers participation, besides the management factors.