Noun phrases in Japanese are typically accompanied by postpositional particles. According to the literature, these postpositional particles can be divided into two subclasses: case markers such as the nominative ga and the accusative o, and postpositions such as kara ‘from’ and e ‘to’. Given that the differences between case markers and postpositions would not be quite obvious to children acquiring Japanese, an acquisitional question arises as to whether Japanese-speaking preschool children are sensitive to this distinction. This study builds on and extends the two previous studies of my own (Sugisaki, BUCLD 35 Online Proceeding Supplement, 2011; Language Acquisition 23, 75–88, 2016), and presents evidence both from an experiment and from an analysis of the spontaneous speech data which argues for the view that the relevant distinction is already in the grammar of Japanese-speaking preschool children. The findings reported in this study are in conformity with the view that child language acquisition is achieved with the help of innate faculty of language.