Learners’ beliefs about language learning are considered very important, yet their developmental nature still remains mostly unexplored. The purpose was to examine how Thai students change their beliefs about language learning after learning the Japanese language in an 11-week elementary-level course. The study also explored the relationships between the learners’ beliefs and their achievements in language learning. Undergraduate students in a Thai university ( N = 73) completed a modified version of Beliefs About Language Learning Inventory (BALLI) developed by Horwitz at two different times; one before taking the Japanese language course (pre-course) and another after having completed the course (post-course). A significant increase at the dimensional level was identified in one of the five conceptual belief dimensions, while changes were also observed in another factor yet it was not statistically significant. Pearson product-moment correlations between the belief factorial mean scores and the participants’ Japanese and English achievement levels were not very strong, and most of them were not statistically significant. It appeared that the students’ experience of studying the Japanese language were related to the change of their beliefs about language learning in some factors, implying the beliefs’ mixed and complicated nature: some categories of the beliefs are modifiable and developed through learning, while others stable and unchangeable.