Teachers' self-efficacy is called teacher efficacy and has proved to be powerfully related to many educational outcomes such as students' achievement and motivation, as well as teachers' enthusiasm and commitment to teaching. This study investigated what factor influences teacher efficacy among English teachers in Japan. One hundred and eighty-nine English teachers responded to the questionnaire which consisted of five scales: teacher efficacy scale, teachers' English proficiency scale, collegiality scale, teacher development scale (which consisted of five subscales: academic development, English proficiency training, reflective practice, development perception, and reality adaptation), and teacher belief scale (which consisted of four subscales: learner-centered communication, ideal acquisition, human education, and limitation). The results showed that teachers' English proficiency had the strongest impact on teacher efficacy. Teachers' adaptation to the actual school circumstances and reflective practice also demonstrated relationships with teacher efficacy. Results also indicated that teachers' belief in learner-centered and communicative teaching had a negative impact on teacher efficacy.