Multicast is a message delivery scheme from a source node to all nodes that belong to the given membership through a multicast tree. This paper discusses how source based multicast trees are set up such that the maximum damage due to failure of any link to the multicast trees is minimized. Then, this paper proposes a multiagent based method for finding such multiple source based multicast trees. The proposed method assumes that candidates of multicast tree rooted at each source node are given according to some criteria. Under this assumption, first, shared links in multiple multicast trees whose source nodes are different are recognized. Then, a multicast tree rooted at each source node is determined such that damage due to failure of a shared link is minimized. The proposed method has a potentially good adaptability for change of both network resources such as topology and link capacity and resource requirements such as source node, membership, and bandwidth of multicast trees.