This study examined the effect of class sizes on differences in the trajectories of elementary school students' long-term Japanese language achievement by analyzing panel data composed of standardized achievement test scores at five points from around the end of first grade to around the end of fifth grade. The data for 103 schools, 162 classes, and 3,460 students were analyzed. Multilevel analysis with a model postulating three levels (student and time, student, and school) was conducted. The results suggested that smaller classes are advantageous for students' Japanese language achievement trajectories.