The purpose of this study was to investigate how 4-year-old children change their relationships by teacher's presentation of play, and how teacher's presentation of play has an effect on changes in their relationships. They were investigated observational data, focusing on "sumo play" of 4-yearold boys, which were teacher's presentation of play. It was found that the boys were developing mutual relationship although the relationship was constrained by ego centric thought. In addition, as the boys shared an interest of "sumo play", the interaction with them increased. The class teacher watched the boys' interaction, spoke for them when they had a quarrel and prompted them to interact, which seemed to support the development of the boys' relationship.