Voice is the natural communication medium between humans, and so a software agent that can communicate user with conversation in Japanese, is applied for a prototype CAI (Computer Assisted Instruction) system. The developed agent-based CAI system is an instructing system that the agent teaches students by vioce speech with showing the graphs, figures, tables, pictures, and text on the display, and asks questions whether the students¡¡understood the given contents or not. The student can ask questions at any time during the lesson, even by interrupting the lecture of agent. The subject of the CAI system is the knowledge of nuclear power generation. The trial and the evaluation of the developed CAI system was made by questionnaires of several student, and it was found that the teaching by the voice communicating agent was more friendly than by conventional text based one, but there remained some problems in conversation. Based on the experience, more improved scheme of the agent interface were discussed in this paper.