A research question of this paper is about how on-line political groups are successfully formed and maintained. Given the low cost of participation the organization of an on-line group looks fairly easy. However, as the low cost helps people to 'exit' easily it does not look easy to successfully manage an on-line group. In this regard, 'Nosamo'(People who love Roh Mu Hyun) attracts special attention. It was formed by 7 people in 2000 but now the number of its members amounts to more than 100 thousand in 2004. This paper suggests that the reasons for this successful internet group activity are two-fold. One is that its members share a common political identity for political reforms. That is, it provides a common purposive incentive. The other is that Nosamo has succeeded in providing members with solidary incentives that provokes fun and interests, and it helps strengthen networks, solidarity and friendship and ultimately widen mutual understanding. This signifies much to political parties in South Korea, which lack voluntary activists and supporters.