In this paper, I demonstrated that there must be enough working memory capacity for Japanese children to correctly comprehend case-markers. Case-markers represent the relationship between a noun and the predicate, and this relationship cannot be determined until the predicate is given. In other words, if the predicate is not realized, case-markers are retained as phonetic strings. It is generally considered that phonetic strings are retained in the working memory, and the accuracy of the retained information depends on the working memory's capacity. We predict that children's ability to retain the phonetic strings of case-markers is dependent on their working memory capacity, and hypothesize that children with insufficient memory capacity cannot utilize case-markers to interpret a sentence due to the inability to retain the phonetic strings accurately. We conducted a listening span test to assess the working memory capacity, and a picture selection task to examine children's interpretation of a sentence. In the latter test, we used scrambled sentences of the [NP-o NP-ga V] form to test the children's comprehension of a sentence with case-markers. To examine whether retaining case-markers as phonetic strings in the working memory imposes a significant load on comprehending scrambled sentences, we employed two types of scrambled sentences with the accusative and nominative markers dropped respectively in each type. If the retention of the case-markers' phonetic string is a crucial factor, it is predicted that children can comprehend these dropped patterns correctly. The results show that the above holds for children in the high memory span group, and support the claim that there must be sufficient working memory capacity in order to comprehend case-markers correctly.