Purpose The purpose of this paper is to develop bansho analysis as a research method to improve observation and analysis of instruction in lesson study, which could potentially visualise pupils’ thinking processes in a lesson. Design/methodology/approach The paper opted for a qualitative method of case study analysis. Data are drawn from a Year 6 Japanese Language lesson of a Japanese primary school. Data collection and data analysis are informed by transcript-based lesson analysis. The process of bansho formation is also reproduced based on video and audio data. Findings Bansho analysis illustrates three main patterns of pupils’ thinking processes, namely, variation of ideas, connection of ideas and attention to ideas. Pupils’ opinion sharing at the beginning of the lesson led to a variety of ideas and they were recorded as part of bansho. There, pupils proceeded to establish connection among ideas. Finally, pupils displayed attention to ideas which were recorded on bansho, by returning to ideas that intrigued them. Research limitations/implications There is a need to investigate the teacher’s role in bansho formation processes in order to develop a more comprehensive bansho analysis method. Other teaching and learning materials such as lesson plans and pupils’ notes should also be included in the study of bansho to develop a more comprehensive bansho analysis. Originality/value Bansho analysis proposed in this paper allows educators and researchers to study bansho with visualisation of bansho-related data. It would serve as an invaluable source of evidence during the observation and reflection stage of lesson study cycle.