The experience of transferring from one culture to another during childhood can represent a crisis of identity for some individuals. "Third Culture Kids" are children who grew up outside their parents' culture during their developmental years, leading them to develop a third cultural perspective. Previous studies have reported that the experience of transferring to different culture is often accompanied by identity-related difficulties. Ethnic identity is an aspect of collective identity that plays a particularly important role among members of cultural minority groups. Stable ethnic identity can help members of cultural minorities form identities and maintain stable mental health. Family relationships also play an important role in identity. The current study had two main aims: (1) examining the relationships between ethnic identity, identity scale scores, general health questionnaire scores, and background factors, and (2) examining family relationships among people in Japan who experienced a cross-cultural transition during school age. The results revealed two important characteristics for adaptation and the development of identity: experiencing cross-cultural transition during early school age (6-12 years old) affect ethnic identity development as Japanese, and stable environment, including family relationships, and having a concept of ethnic identity from early childhood.