The aim of this study is to examine the process by which supporters of children with disabilities acquire expertise in the early stage of their careers in child development support centers. Interviews were conducted with a supporter of children with disabilities who had worked in a child development support center for three years. The results indicated that the supporter developed expertise by accumulating experience in, and reflecting on, specific duties such as “interacting with children,” “interacting with family members,” and “interacting with other childcare institutions,” and by learning from the ways in which other personnel perform their duties. It was also found that supporters experience endless and repeated cycles alternating between establishing their professionalism and doubt in the process of deepening their professionalism. For example, they may experience doubts about their expertise in “interacting with family members” once they come to realize that their expertise in “interacting with children”
has developed.