This article explores learner language of Japanese EFL students by analyzing linguistic errors in their essays and investigates the significant difference of error types by their levels of writing proficiency. The collected errors were categorized by error types to compile frequent error types characterizing different proficiency levels. The results show that the most common errors for all the students are related to wrong words and sentence structures. They also show that the elementary-level students frequently make spelling errors, while the intermediate-level students often make verb-related errors. The case study presented here contributes to a better pedagogical approach to promote Japanese EFL students’ accurate writing skills evidenced by error analysis from a comparative linguistic perspective.