The case marking phenomenon in Korean is closely related to semantic entailment and should be studied as an interface between syntax and semantics. Assuming a direct mapping between syntactic realization and semantic information, this study investigates the role of the accusative marker ul/lul in Korean, and explores the semantic constraints working on the argument realization on the basis of proto-roles hypothesis. Specifically, I will study various types of case alternations in Korean and clarify the role of the accusative marker ul/lul, which manifests the distinction between direct object NPs and oblique NPs.