How do school-based and institution-based teacher educators in Japan acquire their identities as teacher educators? What are the turning points and difficulties in this process for them? In order to address these research questions, the authors of this paper conducted peer interviews with four teacher educators who have experience as school teachers. These interview data were then analyzed with a qualitative approach using SCAT proposed by Otani (2007). Findings showed that the process of acquiring professional identities of the four teacher educators could be divided into the "environment dependent" type and the "proactive transition" type. The "environment dependent" type was triggered by being given the job as a teacher educator; the "proactive transition" type was triggered by the voluntary awareness of being a teacher educator. Findings suggested that the "environment dependent" teacher educators who belong to schools and administration agencies need support especially in the introductory period. Furthermore, it was suggested that in order for "environment-dependent" teacher educators to establish their professional identity, it is important to establish a personnel system that allows them to anticipate transfers and an induction program that includes a mentoring program.