To achieve a smart sensor network with Web-based sensor nodes such as Field Servers, we propose a hierarchical agent system that manages sensor nodes autonomously and flexibly, and acts as an intelligent module of the sensor network. Our agent can treat Web applications in addition to sensor nodes and is a Web application itself; thus, it can control other agents hierarchically. This hierarchical structure provides the Web-based sensor network with high scalability and robustness. The leaf node of our agent system, called the Field Server Agent Box, is equipped with an agent system, a Web interface, a storage function, and data viewers. It can be used with Web-based sensor nodes at sites where an Internet connection is not available. In this study, we also examine the leaf node?s scalability.