Argumentation and dialogue provide important foundations for agent-oriented computing since social computing mechanism such as negotiation, cooperation, conflict resolution, etc. are to be built on them. In this paper, we consider relationship between argumentation and dialogue in a formal way. In doing so, we first describe a primitive but natural inter-agent dialogue model in which dialogue can be seen as partial argumentation that allows us to avoid excessive or unnecessary conflicts. Next, we give the semantics for such a type of dialogue, and the soundness and completeness under it.