Music teachers in schools are required to possess diverse qualities. Their professionalism is often conceived as a list of objective knowledge and skills, such as piano accompaniment for textbook songs, coaching for performance improvement, knowledge of curriculum guidelines, and even understanding of traditional Japanese instruments. However, this leads to “the difficulty of objectifying professionalism", as declarative knowledge does not always translate directly into practical teaching, nor do specific procedural skills consistently function across diverse classroom settings. Furthermore, defining professionalism for music teachers is complicated by the unique and inherently undefinable nature of "music" itself, a challenge termed "the difficulty of defining music". Even academic disciplines struggle to definitively answer "what is music?". These two fundamental aporias prevent a clear and actionable definition of music teacher professionalism.Given the impossibility of objectively defining music, it is equally impossible to reduce the professionalism of music teachers in school to a mere list of abilities. Therefore, "professionalism for music teachers" must be described from a social-constructivist perspective. This paper proposes unlearning as a potential breakthrough for these challenges. Unlearning, rooted in a social-constructivist epistemology, shifts the focus from merely acquiring a boundless list of objective knowledge and skills to a dynamic process of deconstructing and reconstructing one's understanding. By embracing the "fluctuation" inherent in music, unlearning allows for a professionalism that prioritizes the continuous inquiry into "what is music?" and "what is good music education?" over fixed answers. This perspective suggests that true professionalism for music teachers in school lies in this ongoing, critical re-evaluation of their beliefs and practices, and the ability to continuously unlearn.This study used semi-structured interviews with multiple music teachers, and analyzed the narratives within the TEA framework, focusing on TEM”. TEA/TEM depicts life course trajectories within irreversible time and analyze their beliefs and values. The TEA was developed in the late 2000s by cultural psychologists and psychologists using qualitative research, and is currently used nationally and internationally in psychology and related fields in Japan.TEA consists of three methodological theories: Historically Structured Inviting (HSI), Trajectory Equifinality Modeling(TEM), and the Three Layers Model of Genesis(TLMG). Among these, TEM is a method that deals with human experience and draws a model of the process of acculturation and life paths that exist over time for humans. The purpose of this study is to qualitatively describe music teachers’ unlearning processes using the TEA and to articulate a social-constructivist account of professionalism.