A large-scale sensor network often requires a network wide group key for confidentiality of control messages and data reports. The group key should be updated when a certain sensor node is compromised by adversaries in a hostile environment. However, key update messages will consume much energy during hop-by-hop delivery to each sensor nodes. In this paper, we propose a new group key management scheme which minimizes communication overhead of rekeying messages. To achieve this goal, we first give detailed analysis of communication overhead of the existing logical key tree scheme. Then we present the topological key hierarchy scheme which explicitly considers sensor network topology during key tree generation. We show that our scheme can drastically reduce communication overheads of the tree based group key management in wireless sensor networks.