Object relative clauses have traditionally been thought to be more difficult than subject relative clauses in child English. However, recent studies as well as Japanese data show contradictory results. This study disclosed preschool children's superior performance on object relative clauses in Japanese; however, this dominance disappeared for the children who could use both the nominative and accusative case markers as cues for the comprehension of single-argument sentences. Assuming a filler-gap dependency for the relative clause formation, we suggest that there is no difference in the difficulty between subject and object relative clauses in the grammar of Japanese-speaking children.