The objective of this case study was to identify the role of finger gestures in learning mathematics informally during play, especially in sociocultural settings. A mathematical activity involving addition was qualitatively analyzed at a Japanese preschool. We explored how the process of subjectification and objectification contributed to a mathematical activity at a Japanese preschool and how the role of preschoolers' finger gestures changed during the process of learning mathematics. We utilized Radford's theoretical construct of joint labor and analyzed Japanese preschool children's mathematical behaviors from a sociocultural perspective. The subjects were 15 Japanese preschool children and their teacher. We relied on both Radford's methodology and a microgenetic approach for the analysis. We found that subjectification and objectification proceeded in the scene of the conversations regarding addition; observing joint labor in a classroom activity offered valuable insights into these processes. In the activity, the children actively extended their practice of posing and answering quizzes, and learned how to resolve a conflict with the support of the teacher. Secondly, although the role of finger gestures was originally used to obtain correct answers to quizzes, it was reconstructed to solve the conflict between the children through the teacher's mediation. This showed that, even in an environment where children implicitly learn mathematics, they learn from one another, including the teacher, and that gestures in mathematical communications function well for developing mathematical thinking and skills.