Languages vary as to how they express possessive relations in morphosyntax. The Slavic group of languages, for example, expresses those relations in the copular BE construction featuring a preposition. There is another group of languages, including Latin, in which the HAVE verb is employed for the possessive expression, though Dative Case in a copular BE sentence is the unmarked strategy. This phenomenon is well-known as the be/have alternation, and has been extensively studied in the generative literature since the beginning of the 1990s. Turning our eyes to Japanese, it has long been held in virtually all known studies or textbooks that it does not exhibit such an alternation. Possessive relations are expressed with the aid of two distinct existential verbs aru and iru, the choice of which is fundamentally governed by the (in)animacy of the Nominative-marked DP. The purpose of this paper is to argue that Japanese is among those languages which exhibit an alternation between HAVE and BE, in contrast to the prevailing view. Specifically, I first give a critical review of the two previous approaches to possessive and existential sentences with aru and iru, and show that neither of them is tenable. I will then propose a morphosyntactic analysis based on the assumption that a phonetically empty postposition and the existential iru are combined into the possessive aru in the course of the derivation, demonstrating that the proposed analysis is empirically superior to the previous ones.