This study focuses on those who participate in the environmental movement and explores factors which are necessary in order for them to stay, or to actively participate within it. The questionnaires were distributed at the annual meeting of an environmental volunteer group, Eco-League, and via mail to those members absent. Factors were categorized as collective benefits, which are related to the achievement of collective goal, and selective incentives, which affect only those who participated. The analysis of 206 questionnaires showed that organizational identification and subjective norm had a significant effect on intention to stay and willingness to exert efforts, while the perceived seriousness of environmental problems and the efficacy of the movement did not. The result indicated that selective incentives, rather than collective benefits had a stronger influence on behavioral intentions. An analysis of social network revealed that concern for environmental issues amongst friends increased participants' commitment to the movement and toward adopting environmentally-conscious behaviors.