This paper investigates whether or not presence or absence of a post-FOCUS Minor Phonological Phrase boundary in Tokyo Japanese is conditioned by grammatical factors such as the presence or absence of a syntactic boundary and a pitch accent. Each boundary of a Minor Phonological Phrase (henceforth Minor Phrase or MiP) coincides with an F0 rise from the initial syllable to the second syllable. The result of our production experiment shows that there is always a significant F0 rise (i.e. a MiP boundary) at the onset of a post- FOC word as long as (i) an XP boundary or (ii) a pitch accent is present. That is to say, only when conditions (i) and (ii) are absent, the MiP boundary disappears at the onset of a post- FOC word. In this way, post-FOC dephrasing in Tokyo Japanese is not arbitrary but governed by grammatical factors.