Abstract Name : Syafira Yunidar Student No. : 1509200080018 Department/Program : English Education/Postgraduate Title : The Effect of Japanese Language Learning on The Learners’ First Language in Indonesia Keywords: First Language (L1), Foreign Language (FL), Effects of FL on L1, Japanese, Indonesian This study investigated the effects of learning Japanese as a foreign language toward the learners’ first language use, Indonesian. The goals of the present study are to examine the effects of foreign language learning on the learners’ first language use and to figure out why foreign language learning affects learners’ first language use. Five Japanese learners who have different backgrounds of Japanese learning were chosen as participants in this research. Three instruments were used in this study; observation, short video, and interview. Observation and short video were done to examine the effects of foreign language learning on the learners’ first language. The interview was done to figure out why foreign language learning affects learners’ first language. The results are; first, the learners showed foreign language effects; grammatical aspects and borrowing. Second, four factors why foreign language learning affects learners’ first language were found in this study, i.e. personality, motivation, experience and environment. In short, learning a foreign language (FL) influences learners’ first language (L1), in this study, learning Japanese language affects learners’ first language, Indonesian. Correspondingly, the previous studies done by Pavlenko (2000, 2003), Jarvis (2003), Wang (2014), that learning a foreign language affects learners’ first language.