In the next generation mobile environment, which uses 4G cellular and high-speed wireless LANs, novel group communication services among mobile users are expected to grow up. Security technologies for these group type services are challenging research area, especially, decentralization of group key management is important for large users. In this paper, we propose a fully decentralized key management scheme FDLKH that provides a key updating mechanism for dynamic group without any central server. This scheme inherits the key updating mechanism of the Logical Key Hierarchy scheme LKH, which is based on a central server system, and extends the LKH not to expect any central server but to use representative members of a group called captains. On the FDLKH, the total variety of keys in a group is half of that of the LKH. The costs for a member join or leave keep the logarithmic order of the number of members.