This paper claims that the syntactic category of Japanese relative clauses can be larger than TP. The fact that Focus Particles can be found within Japanese relative clauses shows that the licenser, Focus-head, should be located at the CP-zone, adopting Rizzi's (1997, 2004) cartographic analysis, on the assumption that Focus Particles can be licensed in situ. However, not every relative clause is larger than TP. We will present new data on Nominative/Genitive Conversion, which lead to a generalization that Focus Particles cannot appear in Japanese relative clauses where genitive subjects are allowed. This can be explained straightforwardly if we assume that Focus Phrase is missing in this type of relative clause. We will try to explore the mechanism of the case alternation phenomenon, with the refinement of Saito's (2004) dichotomy of T (declarative T and adnominal T). © 2011, The English Linguistic Society of Japan. All rights reserved.