現在のほとんどの音声対話システムは，ユーザとシステムの １ 対 １ の対話を扱っているが，我々はシステム側のエージェントを ２ 人にした三者対話システムの開発を行ってきた．対話ドメインは従来の 「うどんとラーメンのどちらが好きか」 の他に 「北海道旅行と沖縄旅行のどちらに行きたいか」 を追加し，それぞれ同じドメインの二者対話システムと三者対話システムをユーザに使用し，評価実験を行った．被験者実験の結果，三者対話システムは，対話の弾み具合の向上や，対話の雑談らしさの印象を被験者に与えるという結果が指示された．Almost all current spoken dialog systems have treated dialog that one user talks with one agent. On the other hand, we investigated the multiparty dialog system which treated two agents. We developed the three person's dialog system and two person's dialog system in one domain, which treated the same dialog tasks "Which do you prefer udon and ramen ?". We also developed a new dialog system in another domain, "Which do you prefer trip to Hokkaido(snowy region) and Okinawa(tropical region)?, and compared user's behavior/satisfaction. According to the results of the experiments, the three person dialog system performed better in terms of lively conversation, and user can talk with the agents more like chatting.