The exact structural status and feature composition of Japanese functional postpositions such as 'ni', 'de', and 'to', which can follow the other postpositions and behave differently in various usages, are still in question. Modifying some proposals in Ayano (2001), I claim that these grammatical postpositions, which show complementary distribution, form an endocentric structure involving lexical postpositional phrases. I propose that they are functional postpositions 'p' by using Van Riemsdijk's (1990) Categorial Identity Thesis (CIT), contrasting them mainly with the pure lexical postpositions 'P'.