We examined the associations and likely pathways underlying the relationships between peer attachment style, self-concept, and Internalizing/Externalizing Problems among high school students in Japan. A total of 228 senior high school students (186 boys and 82 girls; mean age = 16.4) completed the Attachment Questionnaire for Children, Self-Description Questionnaire II-Short, and Youth Self-Report. The main results were that securely attached adolescents reported fewer mental health problems and more positive self-concept than those who reported insecure attachment. Some patterns of associations among variables appeared to be different across gender. The Structural Equation Modeling provided a support for the mediating role of self-concept in influencing the relationships between Attachment and Internalizing Problems, but not Externalizing Problems. The paths for the model were significant across gender. The results promote understanding of psychological processes that influence the relationships between attachment and psychological well-being among high school adolescents in Japan.