In this paper, a playmate robot system, which can play with a child, is proposed. Unlike many therapeutic service robots, our proposed system is implemented as a functionality of the domestic service robot with a high degree of freedom. This implies that the robot can use its body and toys for playing high-level games with children, i.e., beyond therapeutic play, using its physical features. The proposed system currently consists of ten play modules, including a chatbot, card playing, and drawing. To sustain the player's interest in the system, we also propose an action-selection strategy based on a transition model of the child's mental state. The robot can estimate the child's state and select an appropriate action in the course of play. A portion of the proposed algorithms was implemented on a real robot platform, and experiments were carried out to design and evaluate the proposed system. © Springer-Verlag 2013.