Oversupply of post-doctoral fellows (postdocs) has become an increasingly serious problem in Japan, especially after the policies implemented in 1990’s to expand graduate education to produce advanced professionals. This paper, focusing on the career formation of postdocs, analyses why postdocs do not, or cannot become advanced non-academic professionals and stay as postdocs.
　 The investigation is based on two research results on theoretical physics majors (nuclear and particle physicists). One was conducted to interview postdocs themselves, and the other was carried out sending questionnaire to Ph. D. holders who had decided to pursue their careers in the fields other than academic posts. The reason why we focused on theoretical physics was because it is a basic research field whose achievements do not lead directly to employment in industries, and that the field has been facing the unemployment problem for a long time in Japan.
　 Based on our research results, we found three phases that are confining their careers as postdocs, and two conditions necessary for their career change. Also in conclusion we made suggestions for initiatives to tackle the situation based on our analyses.