Making a truly useful massively multi-agent system is difficult since the actions of the full ensemble of agents cannot be controlled by designing just one agent. It is critical to control all the agents by using protocols that describe the interaction of agents and the environment in a top-down approach. We introduce a system that uses interaction protocol descriptions and has the capability of controlling hundreds of thousands of agents. This makes it feasible to realize a mega-scale navigation system that can assist the inhabitants of a small city. In developing a massively multiagent system, protocol design and agent development need to be separated to allow specialists to work in concert with one another while honing the different technologies. As a platform for mega-scale navigation system, we devise an architecture for multiagent platforms where the execution of agents scenario and the implementation of agents are explicitly separated. This paper also gives the evaluation and an application example using the platform.