In this paper, we propose a new group rekeying method for changing the multicast group key: Matrix-Mask based Group Rekeying "MMGR." In MMGR, the group controller constructs the rekeying message using a numerical matrix and the group members apply secret bit masks on the matrix to get the group key. MMGR does not need computationally expensive encryption of the group key before transmission and hence eliminates the need to decrypt it after receiving the rekeying message. In MMGR, the computational cost for group rekeying is constant with respect to the number of group members up to the capacity of the matrix used, and the rekeying message construction and the group key retrieval processes require simple logical operations.