In this study, we analyzed the co-reflection of pre-service teacher community in three years of post-class discussions (PCD) on the basis of cultural historical activity theory in the case of an biological laboratory class program. We figured out the sociocultural elements of co-reflection of community using activity system suggested by Engestrӧm. We also analyzed the aspect of reflection in terms of both 'the diversity of reflection subjects' and 'the practicality of reflection.' Finally, limitations of co-reflection were sought in a process of continuos circulation of reflection and practice. The results show that the sociocultural factors of 3-year-PCD changed, and in these changes, 'regular PCD,' 'introduction of tools for facilitating reflection,' 'transition of the subject in PCD from researcher to pre-service teachers' are remarkable. And in these process, co-reflection made positive progress since the content of reflection became diverse and practical. However, in some subjects such as 'managing time,' 'participation of students,' 'mentor's role (a kind of teaching manual),' reflective practice have not developed and similar patterns of discussion were repeated. From these results, we suggested ways of facilitating pre-service teacher community's co-reflection from the viewpoint of an activity system.