Cross-linguistic influence is a systematic and unavoidable feature of language learning. This study aimed to investigate whether prepositive word order in Japanese noun clauses can serve as a cross-linguistic influence element in the acquisition of relative clauses (a form of a postpositive modifier) in English. To test this hypothesis, this study conducted an eye-tracking experiment with native Japanese speakers and showed the following results statistically. First, Japanese participants generally paid more attention to modifier clauses than the noun phrases they modified when reading English sentences with relative clauses. Second, when interpreting accusative relative clauses, longer fixations were observed both in the modifier clauses and in the antecedents, likely due to their complexity for Japanese L2 learners of English, as evidenced by the lowest accuracy rate. Finally, and most notably, the findings revealed that the participants who had not fully acquired the relative clause construction focused more on the left side of the relative pronoun than those who had. This tendency was most prominent when interpreting subjective relative clauses, a result that supports the hypothesis that prepositive L1 modifiers could be a CLI element in the acquisition of English postpositive modifiers.