In contrast to Indonesian, in Japanese, there is a verb conjugation to make a causative sentence. Besides that, there are post-positions (Joshi) that have an essential role in perfecting sentences in Japanese. This makes students studying Japanese unfamiliar and finds it difficult to learn. Various errors in using a causative diathesis are often found. This study discusses what mistakes students experience when using causative diathesis. This research is qualitative research with a descriptive analysis method. Sources of data are the essays of twenty senior students of the Japanese Study Program, Faculty of Cultural Sciences, University of Sumatera Utara, and the Japanese Study Program, Faculty of Language and Communication, Universitas Harapan Medan. The data are errors in the use of causative diathesis found in sentences in student essays. The results showed that there were various lexical and grammatical errors in the use of causative diathesis in the essays of Japanese language students in Medan. Based on the research results, it can be concluded that the errors are (1) errors in the use of joshi ni, ga, and wo in sentences, (2) lexical errors in the formation of causative verbs, and (3) errors in determining the subject of causative verbs in sentences.