I provide a unified account of a constraint on adjunction observed in three complex predicate constructions in Japanese: (i) restructuring motion verb constructions, (ii) light verb constructions, (iii) infinitives with wasure- ‘forget’. It is shown that adjunction (i.e. adverbial modification, adjectival modification, and quantifier raising) in the lower projections are impossible in these constructions. To account for the constraint on adjunction, I propose that (i) lexical verbs (Vs) are phase heads and (ii) adjunction within verbal and nominal domains is constrained by Case.