The purpose of this paper is a corpus-based error analysis of a subordinator "because," using two kinds of electronic learner corpora: The Japanese EFL Learner (JEFLL) Corpus and the Japanese component of the International Corpus of Learner English (ICLE-JP). The former is a collection of argumentative and narrative essays by Japanese junior high school and senior high school students, and the latter is a collection of argumentative essays by Japanese college students. The analyses were made exploring the following three questions: (1) How is "because" distributed according to academic years? (2) How are sentence-fragments with "because" distributed according to academic years? (3) What are causes of learners' misuse of "because"? The following results were obtained: (1) Frequency of "because" was in inverse proportion to academic years. (2) Most of "because" in the sentence-initial position brought sentence-fragments which lack its principal clauses. (3) The misuse of "because" may be caused by L1 transfer, the confusion of spoken and written English, and the influence of junior high school textbooks.
