This study examined how a ninth grade teacher improved an Algebra I lesson through a lesson study approach. We used multiple data sources to investigate the improvement of the lesson towards student-centered mathematics instruction, perceived benefits of the teacher, and factors associated with the improvement of teaching. The lesson group substantially improved the lesson through appropriately launching and effectively implementing worthwhile mathematical tasks and strategically orchestrating students' work. The teacher improved his teaching skills and reflection ability, changed his views about mathematics teaching, and acknowledged the importance of repeated teaching, expert feedback, and self-reflection in improving his teaching.