This study is entitled “Indonesian Grammatical Interference in Japanese Language Learning JFT Basic A2 at LPK Ganesha Karya Abadi. This article presents the Indonesian grammatical interference that occurs in JFT Basic A2 Japanese language learning. The data was obtained from LPK Ganesha Karya Abadi in January 2024 totaled 23 students. The data was collected by qualitative approach, observation, and note-taking techniques. The collected data was analyzed by the interference language by Weinreich (1979) to look at grammatical interference and the factors that cause it. The result shows that most of interference language occurred in morphology, syntax, word order, and using tenses. Interference in morphology consist 23 data omission, interference in syntax consist 20 data, interference in word order consist 15 data, and interference in using tenses consist 4 data. In total, the results found were 62 grammatical interferences. The majority factor Indonesian grammatical interference in Japanese language learning were 2. The first one was lack of mastery of the target language, which is in this study was Japanese language. And the second factor was structural similarity between languages, which is Indonesian and Japanese.