This paper developed a self-assessment scale for high school students to assess their competencies as outlined in the Global Media and Information Literacy (MIL) Assessment Framework developed by UNESCO, and verified the reliability and validity of the scale. MIL, an integrated literacy based on information literacy and media literacy, is considered to empower citizens’ personal, professional, and social participation. We constructed a scale based on the framework of the Global MIL Assessment Framework and conducted a survey among 958 high school students. The results of confirmatory factor analysis indicated that a second-order model fit well. The internal consistency was within an acceptable range. Additionally, we observed a difference in scale scores based on the difference in the learning experiences of media literacy and information literacy. These results confirm the construct validity of the scale. Furthermore, criterion-related validity was confirmed using Web Access Literacy scale developed by Yamamoto as an external criterion. Based on these results, it can be concluded that the high school student version of the MIL self-assessment scale is generally valid.