The teachers’ professional community has been regarded as playing a crucial role in terms of educational practice and meaningfulness. In a emerging environment under the COVID-19 outbreak, teachers’ professional community is drawing more attention for the growth of teacher expertise in pedagogical methods. Recently, nationwide teacher learning community are often being established, but research that focuses on contradictions and development in the community has been overlooked. This study attempted to analyze the process of the expansive learning in an elementary teachers’ learning community by looking at the contradictions and changes in the activity system of it based on Engeström’s third generation Cultural-Historical Activity Theory(CHAT). For this, the authors selected a teacher learning community for elementary science teaching, called Nanuri, that has been in active since 2016. It was found that, first, contradictions and conflicts played a crucial role as a driving force for changes in activity structure as opposed to the ordinary perception that contradiction is harmful. Second, active use of mediation (tools, rules, division of labor, etc.) is required for the teacher learning community to bring members from peripheral participation to playing a central role. Third, the growth of teachers’ community is related to advancement of collegiality and democratic ethos among members rather than depending on the leadership of a few leading group. The development of teachers’ communities goes together with that of professionalism, and meanings of works and identity. This study attempted to show the educational significance and social contribution of teachers’ activity as well as how the expansive learning has evolved. It is expected that the follow-up research on various teachers’ professional communities would advance the teachers’ professionalism as well as theoretical development of CHAT.