Agent systems have been designed and developed using recent agent technologies. However, design and debugging of these systems remain as difficult tasks of designers because agents have situational and nondeterministic characteristics and because any useful design support facilities have not been provided for designers. To raise the efficiency of the agent system design process, we propose an interactive design method of agent systems founded on an agent-repository-based multiagent framework that emphasizes an important feature of agent design: the use and reuse of existing agents from an agent repository. We propose an interactive design environment of agent system (IDEA) and demonstrate its effectiveness.