The purpose of this study was to elucidate the relationship between the severity of Internet addiction and various media-related applications. The participants were junior high school students between 12 and 15 years old. A total of 529 students (283 males, 246 females) were included. The participants answered Young's Internet Addiction Test (IAT) and a structural questionnaire about their access to electronic devices and applications. An Internet addiction prevalence of 4.3% (95% CI: 2.8-6.5%) was reported in this study, with an additional 26.3% (95% CI: 22.6-30.2%) of participants possibly addicted. The accessibility of gaming devices was significantly higher in male students than in female students. The use of applications for SNSs was significantly higher in female students than in male students. Twitter accessibility was a factor that contributed to Internet addiction in both genders. The prevalence of severe Internet addiction among school students in Japan was 4.3%, and Twitter was the most important factor associated with this addiction. Media literacy must be increased in adolescents and their friends, teachers, and families.