This paper proposes a event-triggered formation control method for a leader-follower multi-agent system. The purpose of the multi-agent system is to add more function to the system by cooperative control of multi-robots. Research of multi-agent system is useful for society. Generally, in conventional research, each agent always need to communicate with neighbor agents. So, communication failure like as cross-talk is likely to occur. Furthermore, energy resource of robot is limited. So, it is necessary to save the energy. To solve this problem event-triggered control is researched. In these research, the control input is generated only transmitted information in triggered-time. On the other hand, in the proposed method, each agent estimate the trajectory of neighbor agents. When error between estimated and true trajectory become over the threshold, information is transmitted. By using the proposed method, it is expected to reduce the number of information transmission.