Purpose: This study aims to clarify the process of students' discussion in a problem-solving lesson developed autonomously by a teacher. It is expected to contribute to the accumulating and theorizing of teachers' professional development in school-based curriculum studies. Design/methodology/approach: A transcript-based lesson analysis (TBLA) was conducted using quantitative and qualitative methods. The data were from a third-year junior high school social studies lesson in which whole-class discussions took place. The frequency of the appearance of keywords in the utterances was represented in the form of cumulative relative frequency graphs, and a diagram of the students' discussion process was drawn based on them. Findings: The diagram confirmed the frequent occurrence, disappearance and transmission of keywords in the process of the lesson. The interpretation of the lesson transcript revealed that the students listened carefully to others' utterances, shared important keywords and examined the ideas critically. They were supported in identifying the connections between their interests by the teacher. The students' interactions and teacher's interventions were facilitated by the integrated curriculum design of teaching and learning. Originality/value: This study verifies that students' collaborative and critical learning takes place in a lesson developed by a teacher based on the practiced curriculum. The findings of this study suggest that lesson study (LS) focusing on the words in utterances provide clues for evaluating the practiced curriculum and reveal the teacher's professional development.