The purpose of this thesis is to propose a mechanism of genitive Case assignment in Japanese in which both a determiner (D) and a complementizer (C) independently play a role. I propose that both D and C can basically assign genitive Case, but C can serve as a genitive Case assigner only when the clause contains a nonfinite T. The evidence for this claim comes from Accusative/Genitive Conversion in Kansai Japanese, which was reported relatively recently by Asano and Ura (2010). What is the most significant in Accusative/Genitive Conversion is that the pattern in which both a subject and an object are marked as genitive at the same time is observed in Kansai Japanese, and it gives us crucial evidence for the claim that two different ways of genitive Case assignment are involved in Japanese.