This study examined whether the choice of interactors is influenced by indoor and outdoor situations. Peer relations among twenty three-year-old, eighteen four-year-old, and twenty five-year-old children in an urban preschool in Japan were observed during indoor and outdoor free play situations. We analyzed the partners in the interactions, the number, and the stability of interactors. The Shannon-Wiener diversity index (H) was used to measure the stability of interactors. In outdoor situations, three-year-old and four-year-old children were involved with a diversity of interactors, while four-year-old children's preferred friends were stable. Five-year-old children showed a relation with stable interactors in both indoor and outdoor situations, choosing different interactors in each situation. In addition, the children who had few interactors in indoor situations increased their relations with interactors in outdoor situations. These results suggest that three-year-old and four-year-old children are affected by environmental factors that seem to stimulate the children's physiognomic perception, whereas five-year-old children make use of the environment. Opportunities for children to encounter various situations and meet various peers may facilitate the development of social relations.