Using action research, this study examined the professional development of teachers through the reflections that kindergarten teachers obtain from children's voices and the relevance between the working environment and teachers' reflection at teacher conferences in Japan. Four teachers participated in the study. Data sources included teachers' perspectives on imitation paper, teachers' thoughts and reflections at conferences and during interviews, visual material, and narrative observations of children's play. Children's voices stimulate teachers' reflection and provide deeper insights about diversity among children. The discussions at the conference about children's voices showed teachers' learning in four aspects: beliefs of play, environmental setting, understanding of children, and child-teacher relationships. In the act of listening to children's voices, each teacher was influenced by their job positions and by the relationship within teacher groups in reflection at teacher conferences. Teachers appeared to understand each other's beliefs by engaging in regular dialogue at conferences. This study contributed to the improvement of in-service training methods in early childhood education and care.