Bansho (Japanese board writing) is becoming an area of interest among researchers and educators as it serves as an asset to make a searching inquiry into teaching. This study aimed to identify the principles of teacher's decision-making in bansho process. Data of the study were drawn from a sixth-grade Social Studies lesson in a school in Japan. The lesson was observed and then transcribed into the lesson transcript. Reproduction of bansho formation (what/how/when pupils' utterances were written on the chalkboard) was also a primary source of data. Then, a comparison between pupils' utterances and bansho content based on a coding scheme was made to understand what was (not) written as bansho by the teacher. From the comparison, three main principles of teacher's decision-making in bansho process have been identified. As the classroom environment is multifaceted, dealing with complex situations involving pupils' ideas and inquiries is a dominant element in a teacher's life. Through an examination of teacher decision-making in bansho process in a lesson, the conditions and purposes teachers are trying to address in a classroom can be better understood. This provides a valuable opportunity for teachers to reflect on their practices, either individually or in a school community.