English-medium instruction (EMI) courses and programs have strategically increased at the tertiary level of education in Japan in order to be internationally competitive. Implementation of EMI aims at attracting international students and improving Japanese students’ English proficiency. On the other hand, shifting the medium of instruction from Japanese to English challenges lecturers in teaching academic content. Nevertheless, they receive little pedagogical training, and thus its impact on EMI faculty is not well-understood. Given this, the purpose of this study is to investigate the effects of EMI professional training on university lecturers who teach courses in English at a national university. Conducting pre- and post-course surveys and follow-up interviews with six lecturers who attended EMI training, the results show that the EMI training helped them to change their conceptualization of EMI and pedagogical approaches. The study suggests that universities should develop professional training to support EMI faculty members.