This study aims to change the pedagogical content knowledge (PCK) of an experienced elementary school teacher from the perspective of autonomous learning using a reflection matrix (Yasumori, 2013). The participant in this study was an elementary school math teacher with 11 years’ experience. The research methods were as follows. The teacher was interviewed while watching a video recording of him teaching arithmetic. He then analyzed himself using a reflection matrix. Subsequently, he was interviewed again, while watching a second video recording of him teaching arithmetic. Following that, he analyzed himself a second time using another reflection matrix. Results revealed the following: (1) by using reflection matrices, the teacher with 11 years’ work experience recognized the learners’ personalities and was conscious of their autonomous learning in the second recording and (2) when the target grade was changed, the teacher’s PCK regarding the learners’ personalities and their autonomous learning remained strengthened even after 6 months.