School-based extracurricular sports activity (SBECSA) provides one of the main opportunities for adolescents to play sports in Japan. However, maintaining active SBECSA is difficult because of the large burden on teachers to manage SBECSA and a lack of SBECSA teachers who can coach expertly. To resolve these issues, the recruitment of external coaches has been promoted. However, the number of coaches and frequency of coaching are not sufficient for the current demand. Additionally, it is not clear how to promote the engagement of external coaches. Therefore, the purpose of the present study was to explore a variety of facilitators and barriers associated with the involvement of external coaches into SBECSA. Twenty-five external coaches were recruited from junior high and high schools across Japan. Data obtained through personal semi-structured interviews were analyzed using the KJ method (a qualitative type of analyses). As a result, seven facilitators (positive feelings, adequate system, positive social support, comfortable climate of SBECSA, environment, growth of external coach, network building), and six barriers (negative feelings, inadequate system, lack of support, uncomfortable climate of SBECSA, poor environment, burdens) were identified. In conclusion, SBECSA would become more attractive for external coaches by enhancing facilitators and reducing barriers.