This dissertation presents the design of a novel joining-in-type humanoid robot-assisted language learning that uses two robots to conduct a goal-oriented conversation with the human learner to practice English as a second language. The system uses implicit learning as the main learning style to teach the usage of a specific expression form. A mix of tutoring and peer learning is implemented in the course of a three-party conversation. This learning style enables the learner to gain linguistic knowledge, and at the same time it improves the performance of the speech recognition engine. / 博士(工学) / Doctor of Philosophy in Engineering / 同志社大学 / Doshisha University