The effect of the CAI system is not so sufficient for the users to master the subject when the students just see or read the learning material passively by the conventional WWW-based CAI. For more effective learning by CAI, a new method of CAI system has proposed applying the educational theory of ``Learning by Teaching¡Ç¡Ç. The theory of ``Learning by Teaching¡Ç¡Ç is that when the learner teaches others actively and the responses be fed back to himself, he can learn more deeply by thinking and self-examining. So the new conversational WWW-base CAI system has provided by the introduction of personified agent applying this theory. The CAI system provides a new system function of teaching to the personified agent ``Virtual Student¡Ç¡Ç. Through the interaction with Virtual Student, the student can promote his learning. Furthermore, to evaluate the effectiveness of this CAI system, a laboratory experiment was conducted and achievement test and the questionnaire was given to the two groups studying by the new CAI or conventional WWW-based CAI. And it was confirmed that this CAI system was effective for learning.