The actomyosin-based contractile ring (CR), which assembles at the cell equator, maintains its circularity during cytokinesis in many eukaryotic cells, ensuring its efficient constriction. Although consistent maintenance of the ring is one of the mechanisms underpinning cytokinesis, it has not yet been fully addressed. We here investigated the roles of fission yeast myosin-IIs (Myo2 and Myp2/Myo3) in ring maintenance during cytokinesis, with a focus on Myo3. A site-directed mutational analysis showed that the motor properties of Myo3 were involved in its accumulation in the CR. Ring assembly was often deformable and not properly maintained under conditions in which the activities of myosin-IIs localizing to the CR were decreased, leading to inefficient cell division. Moreover, Myo3 appeared to form motile clusters on the ring. We proposed that large assemblies of myosin-IIs consolidated the CR by continuously binding to F-actin in the ring, thereby contributing to its maintenance.