The “wall at the age of nine” is used to refer to the difficulty teaching deaf children Japanese case particles. As existing research has focused on verbs with which subjects and objects are symmetrical or interchangeable, it is difficult to grasp whether the children failed to understand the underlying deep cases or the surface cases (i.e., surface forms or case particles). In this study, we therefore conducted a test asking deaf children to choose the illustration that matched the sentence shown to them in their first language: Japanese Sign Language (JSL). The participants were deaf children aged 6 to 15 from Grade 1 of a deaf Elementary School to Grade 3 of Junior High School in Tokyo. The results show that until Elementary School Grades 3 and 4 (children aged 9‒10), their encyclopedic knowledge and recognition of deep cases were not clearly separate, while older children became able to distinguish between the two. We concluded that deaf children have a fair understanding of deep cases and consider that, by utilizing the method of teaching Japanese as a second language to adults, we can develop teaching materials that allow deaf children to match the deep structure to the surface structure.