This paper argues that Nominative-Genitive Conversion in Japanese is allowed only in the clauses headed by the verb with the special verbal morphology called Rentai-kei.
i. [Boku ga/no omou ni] hannin wa zettai ni naibu no mono da.
Based on the generalization, it is proposed that the special verbal morphology is the overt realization of merger of V and a null C and that the V-C amalgam attracts the Genetive-marked DP into Spec-CP for Genetive Case feature checking･
ii. [CP Genetive DPj<GEN> [TP...ti...] C + T + V <GEN> (Special Morphology)]
It is shown that our theory has a number of synchronically and diachronically important theoretical consequences for Japanese Syntax. We also propose a parametrized theory of Case Checking and bring some new insights into the problem of Transitivity Restriction.