During interpersonal interaction, common ground of our experiences takes an important role to facilitate mutual understanding as well as efficient communication. We introduce the concept of common ground, which has been an important issue in psychology, for developing robotic systems. With the conceptualization of the common ground among human, robot, and environmental objects, a comparative study is carried out between Human Human Interaction (HHI) and Human Robot Interaction (HRI), using paired groups with different expertise in "questioning and answering tasks." For comparison with HHI, the performance of a robot in the task was determined by both its own expertise as well that of its human partner. This indicates that the performance of the robotic system depends on the common ground shared with the human partners, and implies that a robotic system is required not only to be intelligently smart but also to be communicatively smart.