This study examined how secondary teachers (junior and senior high school teachers) of English as a foreign language (EFL) in Japan participated in collaborative action research and engaged in their professional development over four years. In particular, little is known as to how EFL teachers challenge their beliefs and implement innovative practices based on communicative language teaching (CLT) through teacher learning processes. Using a mixed methods design, the study identified three developmental stages regardless of years of teaching experience: (1) Challenging teacher beliefs through TESOL classes and modeling teachers who have done similar action research (AR) through trial and error teaching as they practice communicative activities; (2) Making sense of teaching through adapting and modifying theories of CLT; and (3) Building confidence in teaching by actually seeing students change. These stages overlap and are not mutually exclusive, depending on teacher and teaching context. The study reveals teachers' dynamic learning processes and professional development.