In Japanese high school English classes, students are often left to have interactions or perform communicative activities not with a teacher but with other students due to a large class size. In the situation, students are ideally notice their own insufficient utterances in order to carry out
self-initiated self-repair. This study investigated self-initiated self-repair attempts and their effects on Japanese high school learners. Thirty-two Japanese high school students with low-intermediate English ability and a native speaker of English participated in the study, with the native speaker
interviewing the students. The students’ utterances were quantitatively and qualitatively analyzed, and it was found that: self-initiated self-repair occurred frequently and, in general, successfully; error repair was most frequently recorded; the success rate of lexical repair was the lowest. Findings observed during the students’ self-initiated self-repair attempts are discussed, followed by discussion of their possible effects. Finally, suggestions are given based on the pedagogical implications from the study.