One controversial issue in the literature on Japanese concerns the question of whether the surface accusative-dative order of ditransitive constructions is base-generated or derived by syntactic movement. In the light of nominalized clauses in which dative-V and accusative-V idioms are embedded, this article shows that ditransitive verbs project an argument structure whereby dative arguments could be base-generated to either the left or the right of accusative arguments, as countenanced by the base-generation hypothesis for the argument order of ditransitive verbs. Nevertheless, dative arguments are not freely ordered relative to accusative arguments. We argue that with ordinary ditransitive verbs, only the dative-accusative order is available by base-generation, the reverse order being derived via syntactic movement, as conceived by the movement hypothesis. We suggest that the dative position below an accusative argument does not reside in a θ-marking domain, and thus it can be filled only by idiomatic dative arguments that are interpreted without θ-role assignment. The data show that both base-generation and movement analyses are necessary for characterizing the word order variation of Japanese ditransitive verbs.