Background: Formal and informal professional development has played a central role in teachers' growth in Japan for many decades [Collinson, V., and Y. Ono. 2001. "The Professional Development of Teachers in the United States and Japan." "European Journal of Teacher Education" 24 (2): 223-248]. This is especially true for elementary school teachers, who are not licensed in a specific subject area. In Japan, teachers are trained for each school level separately, including kindergarten, elementary, secondary schools, and special needs education [Ministry of Education, Culture, Sport, Science, and Technology [MEXT]. 2018. "Principles Guide Japan's Educational System." Accessed August 6, 2018. http://www.mext.go.jp/en/policy/education/overview/index.htm.]. The lack of in-depth subject-specific training is a challenge for elementary school teachers, who are responsible for teaching all subject areas. For many decades, school districts have used a lesson study approach. In Japan, lesson study (emerged in the 1920s as professional development opportunities for in-service teachers) helps various types of knowledge become more visible for teachers, which might include colleagues' and mentors' feedback and suggestions about pedagogy and students' reflective and critical thinking. This also allows teachers to encounter new or different ideas and to refine their knowledge [Lewis, C. C., R. R. Perry, and J. Hurd. 2009. "Improving Mathematics Instruction Through Lesson Study: A Theoretical Model and North American Case." "Journal of Mathematics Teacher Education" 12: 285-304]. The purpose of this study was to investigate Japanese elementary teachers' professional development (lesson study) experiences using the conceptions of andragogy in physical education (PE) in a metropolitan school district. Participants and setting: Six participants were selected from one elementary school located in a metropolitan city in Japan. There were (a) 2 lesson study presenters, (b) 1 mentor, (c) 1 professional development coordinator, (d) 1 school principal, and (e) 1 school district PE coordinator. Approximately 600 children were enrolled in the school, and there was a total of 33,506 elementary children in the school district. Research design: The research method was descriptive-qualitative, using an in-depth, semi-structured interviewing approach [Seidman, I. 1998. "Interviewing as Qualitative Research: A Guide for Researchers in Education and the Social Sciences." 2nd ed. New York: Teacher College Press]. "Data analysis: A thematic analysis method" [Fereday, J., and E. Muir-Cochrane. 2006. "Demonstrating Rigor Using Thematic Analysis: A Hybrid Approach of Inductive and Deductive Coding and Theme Development." "International Journal of Qualitative Methods" 5 (1): 80-92] was used to explore, describe, and interpret the data. This allowed the researchers to uncover emergent themes in the data. In the conceptions of andragogy, through thematic analysis, the researchers searched for key or meaningful phrases or words that expose the characteristics of in-service teachers. Findings: Explainable by the conceptions of andragogy, three major interrelated and complex themes emerged from the data analysis: (a) "challenges," (b) "vitalness of a mentor's role," and (c) "self-directed learner." The lesson study approach is an effective way for teachers to reflect on their instruction and promote instructional change. To continue to support teachers to improve their quality of instruction, there is an ongoing need to heighten awareness among teachers, administrators, and researchers to ensure that attention is directed to the learning outcomes of children.