The optimization of multicast routing tree is an important issue for efficient multicast communication. Some previous papers have tried to analyze the one-to-many multicast tree and have obtained that the most efficient tree depends on the characteristics of the receivers such as the number of receivers. For many-to-many multicast, on the other hand, the efficiency of the tree has not been analyzed. This paper analyzes how various components affect the efficiency of the many-to-many multicast tree. As a result, it is shown that the most efficient many-to-many multicast tree does not depend on the number of multicast members. The result also tells that the diameter of the tree, the variance of node degrees, and the number of leaf nodes mostly decide the delay performance of the tree.