In South Korea, over 2% of the certified secondary school teachers teach subjects for which they have no official qualification, leading to out-of-field teachers. The mismatch between the supply of teachers by education authorities and the demand of teachers by schools often causes the certified teachers to become out-of-field. Recognizing the harmful effect of out-of-field teaching on students and teachers, the Korean government has implemented in-service education so called “Minor Qualification Education (MQE)” to help out-of-field in-service teachers obtain additional qualifications to teach subjects on demand. The current state of MQE, however, does not gain enough attention of the education authorities. Critics also argue that the quantity and quality of the MQE are far from being satisfactory. Teachers who acquire a new qualification through MQE are reported to experience identity crisis, as they belong to neither of the two teacher groups. The situation requires us to investigate the current state of MQE along with the perception of secondary school teachers. The purpose of this research is to analyze the trends of MQE during the last 10 years (2005~2014) in order to further reveal the professional practice and identity of teachers who are transformed into in-field teachers through the MQE. Utilizing the national database on teachers, we identify the number and percentage of teachers who have acquired new qualification through MQE. We also conduct semi-structured interviews of 7 secondary school teachers and personnel administrators to reveal the transformation process and results of teachers’ professional knowledge and identity. We discuss the distinctive features of teacher professional knowledge acquisition and identity transformation process. We recommend policy alternatives for supporting out-of-field teachers to become in-field teachers. Keywords: out-of-field teachers; in-service education, teacher identity; South Korea