This research aimed to answer the following question: Why does dialogue deepen mathematics learning? In order to answer this question, the study author attempted to define the characteristics of appropriation, which connect the deepening of individual mathematics learning and that of social mathematics learning, by using the sociocultural approach. In addition, the study author proposed an extended model of sign appropriation and use by incorporating the learning community and the other individual learners as dialogue partners into it based on social constructivism in mathematics education. Finally, this paper explored the following research question: In order to deepen the learning of mathematics, it is necessary to have appropriation to compare and transform the original concept of the learner and that of the speaker, and the suitability of this appropriation is secured by dialogue. The author also identifies appropriation and its two characteristics--dynamic composition and mutual composition.