Lessons for developing students' communication skills have been recently introduced to university English education in Japan, and the lesson format has become student-centered. As lessons are given in English and students have more opportunities to practice speaking in English, there are likely to be controversies over the proper balance between the use of the target language (TL) and the first language (L1) in EFL classrooms. However, no clear consensus concerning the relationship between these has been reached yet. The present research investigated the factors that were related to TL/L1 use among Japanese university students. A questionnaire containing background information and a five-point Likert scale of anxiety was filled in by 252 students. Following this, individual interviews with five students were conducted. The results revealed that while the advantages of L1 use for the students were reported, the students had negative feelings about their use of the L1 due to decreasing the contact with the TL and/or increasing peer and self-pressure. Therefore, students should gradually become accustomed to English-only instruction in an unthreatening environment.