In this paper, we propose the extension of cooperation protocol which realizes the autonomous control of QoS and show its effectiveness with implementation and experiments of our agent-based videoconference system. The autonomous QoS control in the agent-based videoconference system means the framework of adjusting its functions/abilities by itself to cope with the changes of user requirements and system resource situations. But, the original cooperation protocol used by the agent-based videoconference system is not enough to perform cooperation closely to use the limited resources effectively. In our proposal, the cooperation protocol is offended in the following points: (1) introducing a cooperation states, (2) extension of performatives, (3) introducing compromise levels. We embedded the proposed protocol to the agent-based videoconference system and experimented the behavior of the system. According to the experiments on this prototype system, we confirmed that the flexibility of the system is improved.