In this paper, we propose a new method to construct a semi-optimal QoS-aware multicast tree on MANET using distributed computation of the tree based on Genetic Algorithm (GA). This tree is sub-optimal for a given objective (e.g., communication stability and power consumption), and satisfies given QoS constraints for bandwidth and delay. In order to increase scalability, our proposed method first divides the whole MANET to multiple clusters, and computes a tree for each cluster and a tree connecting all clusters. Each tree is computed by GA in some nodes selected in the corresponding cluster. Through experiments using network simulator, we confirmed that our method outperforms existing on-demand multicast routing protocol in some useful objectives.