Second language (L2) learners of various native languages frequently overpassivize one type of intransitive verbs, namely, unaccusatives (e.g., disappeared, arrivedn). Previous studies have attempted to address the culprit of this issue without reaching a general consensus on the cause of such errors. This study investigates whether first language (L1) plays a role in L2 English overpassivization errors with intransitive verbs by Japanese-speaking learners. It utilizes the framework of the Unaccusative Trap Hypothesis (UTH, Oshita, 2001) and hypothesizes that learners ungrammatically passivize unaccusative verbs due to L1 lexicon and morphology. It also predicts that learners generate passive errors with unergatives (e.g., cry, smile) in the same manner as passive unaccusatives. The study examined three types of intransitives: English non-alternating/Japanese alternating unaccusatives (e.g., happen in English and okiru/okosu in Japanese); English/Japanese non-alternating unaccusatives (e.g., arrive in English and tuku in Japanese); unergatives. The study analyzed a Japanese learners’ corpus (JEFLL Corpus), consisting of approximately 70,000 words from Japanese junior high and high school students. The findings in this study are that Japanese EFL learners tend to passivize more English non-alternating/Japanese alternating unaccusatives and unergatives than English/Japanese non-alternating unaccusatives. Given the result, it can be suggested that Japanese learners passivize unaccusatives owing to the influence of their L1 lexicon and morphology.