This study investigates to what extent and why some mother-tongue (L1) words were used in Japanese students\u27 speech in English, using the Louvain International Database of Spoken English Interlanguage Japanese Sub-Corpus. So far, the reason for direct mother tongue use in the second and foreign language (L2) speech by English learners is explained solely as being the lack of knowledge of the English expressions. The data shows that there are some examples of L1 use which cannot be explained only by this reason, as well as of some differences of L1 use according to the speakers\u27 proficiency levels. By analyzing the functions of the L1 use in the speech, this study also aims to shed light on how to improve L2 English speech by Japanese learners of English
