The purpose of this paper is to demonstrate that children's use of Japanese case particles obeys the grammatical principles introduced at the earlier stage of language development. In previous studies concerning the acquisition of Japanese case examined through the experimental method, it has been suggested that children acquire the functional use of case particles at around 5 years of age. This study examines the performance of Japanese case particles in children's natural speech in 2- to 5-year-olds within the framework of the theory of generative grammar. Data focusing on the phenomena "case marker drop" is analyzed. In conclusion, this paper demonstrates that 2-year-olds use Japanese case particles in much the same way that adults do and it will be further suggested that their use of case particles obeys the hierarchical structure of the Japanese language as well as that of the semantic structure. (VWL)