Web-based sensor nodes, which have a Wireless LAN and a Web server for remote monitoring, could be used with little effort by users thanks to their easy installation, monitoring, and management. To realize a sensor network with high scalability, fewer legacy problems, and general versatility, we propose an agent system for operating Web-based sensor nodes via the Internet. The agent system handles many kinds of sensor nodes flexibly and uniformly with the agent program and configuration files. Constructing all agent objects based on the Internet enhances the expansion of operation, control, and scale. By making the agent system architecture, algorithm, and implementation, we demonstrate the capabilities and reliability of this useful sensor network.