Mobile ad hoc networks (MANETs), where nodes can communicate without an infrastructure, require all nodes to be cooperative enough to transfer packets for other nodes. MANETs have been studied for decades, due to its characteristics which are infrastructureless and distributed. However, the existence of malicious nodes that drop packets for their resource protection is one of the important issues of the present research. Reputation systems using trust model have been proposed to detect malicious nodes in MANETs. Here, the trust value indicates the degree of reliance. One of the attacks in MANETs is On-Off attack on which attack nodes keep trust above a certain level by repeating forwarding and dropping packets. In this paper, we propose a trust model focusing on node usage against MANETs On-Off attack (TMUMO). In this study, we detect On-Off attack nodes by considering the characteristics of On-Off attack. Through simulation experiments, we improve detection rate of attack nodes and packet delivery rate in MANETs, and show feasibility of our trust model.