In this study, we assessed the invariance in the factor structure underlying English-language proficiency for two groups of adolescent learners in Japan: students in middle school (ages 13-15 years) and students in high school (ages 16-18 years). Language proficiency was measured using the "TOEFL Junior"® Comprehensive test, an assessment designed to measure the English skills of adolescent English learners in non-English-speaking countries. The study results indicate that a correlated 4-factor model corresponding to the 4 language abilities of reading, listening, speaking, and writing best represents the nature of language proficiency in the 2 groups of adolescent English learners. Moreover, the factor structure is invariant across these 2 groups. However, there is a statistically significant difference in performance on the reading construct. The results are consistent across 2 random samples, thus providing confirmatory evidence of model invariance.This study provides empirical support for the current score-reporting practices for the TOEFL Junior Comprehensive test and suggests that the test scores have the same meaning across test takers from middle school and high school.