In this paper, we focus on silence, which appears as a gap or delay in giving a response during a conversation and is one of the most important factors to consider to have a more natural conversation with robots. In the conversation between a human and a robot, silence can be divided into two parts: first, a silence that a human uses for a robot and second, a silence that a robot takes for a human. Therefore, we conducted a conversation test between a human and a robot in order to clarify the following two points: one, whether humans use silence for a robot and two, how silence used by a robot can be interpreted by humans. The results of the experiment indicate that humans certainly use silence for a robot for some reasons. Participants were asked to label the silences in four different types: Semantic Silence, Syntactical and Grammatical Silence, Interactive Silence, and Robotic Silence. As a result of this classification, there were cases where humans used Interactive Silence to be concerned for a robot, similar to that in case of a human conversation partner. It is now clear that humans use and regard silence in a form closer to a human conversation partner rather than a machine partner while in conversation with a communication robot. In particular, we found that sometimes humans use silence in social sense such as Interactive Silence, which is for the consciousness of a conversation partner.