Clustering is a method for hierarchical management of a network. Especially, clustering is often used in MANETs. Johnen et al. proposed a self-stabilizing clustering algorithm which treats a MANET as a vertex-weighted graph. The algorithm has autonomous adaptability against topology changes and weight changes, while it does not consider the stability of clusters. In this paper, we present a weight assignment method that reflects the mobility of each node to its weight so that the stability of clusters is improved. The proposed method makes nodes that move together maintain a cluster by considering mobility groups of nodes. Simulation results show that the proposed method has improved the number of changes in clusterheads of the existing method by 50%.